I personally have not noticed a significant reduction in throttle use on my V8 4WD
You just need to find the right surface and mash the pedal. The engine just sort of makes a low, pulsing chug sound and the truck goes nowhere.
I intentionally stuck my wheels in an icy ditch once in hi range (rears on ice too). Tried to back out and heard the boing of a quick attempt at brake traction control and then the engine dethrottled and just kind of chugged, quietly and rhythmically. I kept the pedal down just to see what would happen, and every so often there would be an attempt to spin the wheel followed by a quick brake pulse and then the no power chugging again. Rinse, repeat. After that, I locked the diff and got right out.
That said, I've tested the acceleration locked vs unlocked with various pedal inputs on snow and ice many times and, most of the time, unlocked will get me up to speed as fast as locked. An exception can be a full pedal mash, which leads me to conclude that people's complaints about what they feel is a safety hazard in VSC engine dethrottling is due either to driving technique or a difference in sensitivity, for whatever reason, of VSC in various trucks. I wonder sometimes if the sensitivity didn't change, say, with the '06 refresh and perhaps also with the appearance of the '08 VSC off switch.
As for which system manages engine output -- I don't think Toyota has been consistent in how they use the terms TRAC ATRAC VSC and it may well be that TRAC is the right term in present time, but in my '05 manual, TRAC is reserved for 2wd and VSC seems to be credited with controlling engine output in 4wd trucks. Some quotes:
Use the center differential lock system if
your wheels get stuck in a ditch, or when
you are driving on a slippery or bumpy
surface. When the center differential is
locked, the vehicle stability control system
is automatically turned off and the center
differential lock and
“VSC OFF” indicator
lights come on because the function that
controls engine performance interferes
with the process of freeing your wheels
-------------------------------------------------
“VSC TRAC” warning light
This light warns that there is a problem
somewhere in the following.
Vehicle stability control system
Traction control system
(two- wheel drive models)
Active traction control system
(four- wheel drive models)