@HanShotFirst For sure! Baal gets pretty bad at 98 compared to diablo or nihl. He's not so terrible that you cant mix in some runs of him every now and then, but yeah a good bit slower.
As far as switching psettings midway through, I definitely do that, as I have described, but just leaving the psetting on p7 isn't very much difference, if you are interested in testing that out yourself and comparing the numbers. Also compare the xp gain of p7 diablo to p8 diablo. At least in modern patches, p8 is no different exp-wise to p7, due to a maximum exp that can be gained from a monster (and baal caps out on p3 for exp). I'm not at all positive this is the same for your version, but I think it is, so give it a test!
Also if doing CS runs, see if you have any shrines close to waypoints in act 1 anywhere before black marsh. These have an 18% chance of being an exp shrine as oppose to 8.2% in most of the later game. Actually again I'm not sure if this is true for old versions either lol. I don't know much about them.
On nihlathak, I sorta want to test him out too, but dont want to give up the rune drops and stormlash chances from CS. If doing nihla, I would try a mercless hdin and slap on a natures peace. That should make the snakes not terrible (due to avoiding nextdelay from not having a merc), and I THINK the runs would be very doable. But again I'm talking 1.14 and haven't tested.
As far as the drops go, for me I feel like its something like:
Baal - poor for runes, poor for tc87s (but better than nihla)
CS - good for runes, almost zero tc87s (but best for stormlash)
Nihla - medium for runes, poor for tc87s
Hahaha. Good luck!