A Diamond Bow has 33-40 base weapon damage. 40% of that is 13.2-16.
So a 40% Jewel will add 13.2 to min and 16 to max damage, increasing it's average damage by 14.6.
You can't get a 15 max, 15% IAS Jewel, since they're both suffixes.
Question answered.
use this calculator
also check WWS under uniques and diamond bow.
for me i like a sheal + nef for knockback. your first concern in HC is survival and not damage.