What player setting is optimal for Pit runs depends on a number of factors. Arguably the most important ones are:
Playing style - a Poison Nova Necro will have much greater relative success at p7 than at p1 (since a poisonnecro tends to kill pretty much the entire screen with a few novas and a few corpse explosions) than a Berserker Barbarian (whose killing method of choice is a single target attack primarily meant for bosses).
Goal - If your goal is to find some Uniques while also finding a good number of charms, jewels and runes, killing lots of monsters at a high player setting (poison necro or lightning fury amazaon at p7) is going to be good. If your primary goal, the thing which you want to maximize, is to find Uniques, period, it's very likely you will want to focus on killing bosses and champions only, on p1. I say "very likely" because these considerations are affected by a number of other things as well, like how much MF you wear and how fast you run (in addition to the "how many monsters do I kill, and how many of each type of monster" thing). Giving a definite answer based on generalizations is probably tricky. Another goal which matters to some people is gaining experience; if you want to get to level 96 or higher, killing boss packs only on p1 will not achieve that goal for you particularly fast. Doing semi-full clears with a poison necro on p7 or p8 would certainly be better for your goal.
In this particular case, where what we know is that someone wants to maximize his chances of finding Astreon's and Tyrael's, the best way to go about this will almost definitely be to focus on killing boss packs as quickly as possible (which pretty much means p1) with as much MF as possible. Consider that bosses and champions are the monsters far most likely to drop you a Tyrael's; these are also the monsters where a higher player setting will not increase your chance of having a Tyrael drop at all. If we assume you lose any time at all by increasing player setting from p1 to p3 (or p5 or p7), then you would increase your efficiency by not playing at the higher player setting. If we assume you DON'T lose any time by playing at p3 instead of p1 (which I would say is a very very shaky assumption), that means your killing power is great enough that you could most likely wear more MF at the expense of some killing power gear; the end result would be that p1 bosses/champions would still go down (almost as) fast, but you'd wear more MF and thus increase your efficiency. Basically, it's almost impossible to think of a scenario where p1 isn't the optimal player setting for someone who wants to find a particular Unique, when only considering bosses and champions.
This brings us to Minions and Regular monsters. For a higher player setting to be "worth it", you'd have to kill enough minions/regulars, so the increased chance of finding the item(s) you're after from minions/regulars offsets the lost "efficiency" from killing bosses and champions at a slower pace. This gets complicated further by the fact that you can probably wear different amounts of MF if you play on p1 than if you play on p7. Actually quantifying this is pretty tricky, since it will obviously greatly depend between different characters. As a rule of thumb, the more monsters you kill without slowdown, the more likely it is that a higher player setting could be comparable to p1. This can be estimated decently well if you know (=estimate) how many monsters of each type you kill (on p1 and on the higher player setting), how much MF you're wearing (on p1 and on the higher player setting) and how fast your runs are (on p1 and on the higher player setting). If you have estimates of those things, you can get a comparison by plugging them into this
spreadsheet I made some time ago. For most people and most characters, this is probably overkill and just doing p1 runs is probably going to be good. Certain characters (the poison necro and javelin amazon mentioned earlier for instance) are good candidates for where a higher player setting can actually be good though, since they naturally tend to kill more monsters than the boss relatively easily.
Now, the reason everything I wrote above isn't very important in this case is because Tyrael's can't be dropped by regular monsters. That makes it even more of a stretch than before to assume a higher player setting could be better than p1. Basically, now you only have the increased drop odds of Minions to offset the time you lose from killing bosses/champions slower, and there just aren't that many Minions in any given Pit run. I'm not going to say it's completely impossible that a higher player setting could still be good, but I am going to say that even if that's the case, you could optimize your character choice further to where p1 would be better.
In summary, if you want to find Tyrael's Might (and this is the main consideration since Tyrael's is so much harder to find than Astreon's, anyway), you want to kill bosses and champions as quickly as possible with as much MF as possible. This pretty much means doing runs as fast as possible on p1, focusing on those boss and champion packs. Wearing as much MF as possible is definitely helpful, but if it slows you down, chances are that extra MF isn't worth it. An extra second off your average run time is worth a lot more than an extra couple % of MF in terms of your efficiency. This type trade-off can also be analyzed with my spreadsheet, for those interested.
A Lightning Sorc is a pretty good character for running the Pit. If I was in your shoes, I'd fire her up on p1 and see how fast you can (consistently) run. If the bosses take some time to kill, I'd consider dropping some sources of MF in exchange for some more killing power. In general though, I love me some high MF numbers
As for your last question (which I only saw just now) re: blizzsorc in AT vs lightning sorc in Pit, I would guess it's likely the blizzsorc will be slightly more efficient. Answering that more accurately would mean figuring out how long runs take with each character and roughly how many monsters are killed by each character (which is pretty tough/annoying to get a good estimate on, but there tends to be significantly more boss packs in the Pit than in AT). I might be inclined to go with whichever one you enjoy the most, or to switch it up and play one when you get bored of the other. I think either way, both characters are pretty good choices. I'd pay attention to killing speed though, those are some pretty huge MF numbers! Any chance you could save a couple seconds per run in the long run by going to 5-600 instead? Something to consider.
