Serious drag racers use them, so I'm pretty sure they improve times, or they wouldn't waste their money.
I've only owned one car with a high stall speed converter, and I didn't even know it had one. But I never could figure out why it was such a slug out of the hole when ever I would just floor it when taking off.
I'm not one to brake torque my cars except at the drags.
But when I took the car to the drags once, (just to see what it would do) I was pleasantly surprised.
With un-opened headers, no cats, aluminium high-rise manifold, a 4 brl carb (intead of a 2 brl.) with 3.54 gears it ran high 14's. Not bad for a 1977 compact with a stock "cammed" 304.
The SRT with the custom made HSSC, runs 11.7's (I think) and I'm not sure if its owner ever posted anything about his mod, so I'm going to respect his privacy and not "out" him.