Ask a Bowman Thread.

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.

There's no such thing as 0.7 to 0.8 seconds for Strafe. It's plain 720ms. Pierce does not take 1 second to charge. I believe it is now only 540ms to charge. I don't know why you would make some assumption like Ultimate Strafe always hitting Final Attack. It doesn't matter that you take a defense penalty for each shot of Ultimate Strafe, because you take the exact same penalty for each shot of Pierce. And there is no such thing as this "new booster" that reduces Strafe's speed to 600ms. Please refrain from making baseless assumptions in the future.

Strafe does 1288.8%/s with maxed Sharp Eyes. Pierce does 779.4%/s on the first monster and 935.28%/s on the second monster. If it does still take one second to charge, it does 561.9%/s on the first monster and 674.3%/s on the second monster and Strafe is marginally better for mobs of two in terms of DPS but completely and absolutely irrelevant for training efficiency which is dependent on positioning and hits to kill, not DPS.
Maxed Ultimate Strafe does 2353.75%/s with maxed Sharp Eyes, Marksman's Boost, and Final Attack. Maxed Pierce does 1308.3%/s on the first monster and 1473.1%/s on the second monster. If they have reverted the charge reduction on Pierce in Big Bang, which I do not believe they have, Pierce would only do 958.4%/s on the first monster and 1079.1%/s on the second monster, making Ultimate Strafe marginally better in terms of DPS but still irrelevant to training efficiency.

The only situation where DPS on mobs compared to single target is truly worthwhile in noting is in a bossing scenario where targets will take minutes to kill. It is otherwise more efficient to count the average hits to kill and rate of killing on monsters to be trained on; the %/s difference is so insignificantly small, that it will almost never make a real difference in hits to kill with either skill. Strafe will likely take as much time to kill two monsters as it does for Pierce, unless you are over killing with one of the skills.