b321 is on of the best and in my opinion most important KL strings (even more so in buzzsaw for you can hitconfirm in 38% damage with one bar).
Weather you use b312, 11 or b2 depends on the situation.
11 is the fastest starter, but 1 hits high and can be countered with low pokes, d2 etc. The range is limited as well, and I claim you can not hitconfirm 11xxspin (or it's just me...). So overall it's your best starter for punishing strings and specials that are up to about -13.
b312 has good range, hits mid and can be hitconfirmed to deal huge damage. The startup should be about 16f, so I rather use it to whiff punish or at a distance / situation where I can not be poked during startup. So you need good spacing/footsies to utilize it. Technically it is not a "pressure string" - You are at + frames afterwards and unless you have conditioned your oponent to wait for a spinning hat your pressure ends here. I also like to use it as combo ender beacause you get a hard knockdown and cancel into teleport for shinanegans (however, most KL player prefer 21 as Combo ender).
Reagrding the "instant" overhead; it's not quite new to me, but I haven't played around with it either. I have no opinion on that so far, but extra tech can never be useless.