Has anybody here ever gotten a knockout with his burst? I occassionaly get knockdowns but nothing more. 
If bursts are the same as single shot in torso, then it's possible to KO, I once punched guy in NCR to let him grab BA from exit grid, I had low unarmed skill and low % to hit, but I had better criticals perks, 10 LK and x3 MC, got critical hit and knocked him out, so he couldn't grab it... so it's possible to knock out in torso.
Also, how does "Better Criticals" affect bursts?
The same way as any other attacks, gives +20 to critical roll, so you'll have more damage and better effects in torso with critical bursts. Actually it affects bursts good, if torso critical table is the same as Fallout2 torso critical table, it's even better than x1 BRD for SG bursters atleast. The problem is criticals in torso aren't often, so it's better to pick BRDs and MCs instead of BC.