Ask a Bowman Thread.

1. Theoretically would strafe outdo pierce in a mob of 2+ in terms of dps/dpm or would pierce do better in terms of dps/dpm?
2. Will it change after big bang with ul strafe+FA.vs.Pierce+FA?

1. Strafe is 100%x4 every 0.7 to 0.8 seconds (normal crossbow w/booster but no wind booster) while pierce is 850% per mob (plus 1.2x multiplier per mob) and takes 1 second to charge. You'll get 400% to 800% with strafe while pierce does 1700% so no, it's not even close.
Usually you wouldn't use strafe against two or more enemies unless you can down it in two shots. Strafe is a single target attack, not a mobbing skill.
2. If you get FA on both your ustrafe shots (which takes 0.6 seconds w/ new booster) you'd get 1350% per shot or 2700% total. Pierce does 1150% to the first enemy and 1380% to the second plus 150% FA for 2680% total. Since you take a mob defense penalty for each shot, pierce will outdamage ustrafe.
Post-BB, pierce will still be better than ustrafe for multiple mobs unless the mobs are not in a straight line.