Whenever power is applied and the convertor slips (max revs in this condition is called the stall speed) heat is generated since the torque from the engine must be dispersed. The fluid takes this energy and so when loading up the drivetrain with an engine on boost (the whole idea of the technique) heat buildup is rapid. If you do this often I would highly recommend an auxiliary transmission oil cooler (in fact, on 850s it should be a necessity!).

The drop rpm, brake lift, accel drop are all highly related to the running condition of the car, the road surface, the tires, the weather and even more other things.

I drop anywhere from 1400-2100rpm. Sometimes I can start burnouts at 1500rpm, sometimes I need 2100 to get the perfect launch. It all depends.

As for tranny wear, I wouldn't worry that much about it. The torque convertor on the auto 850T is meant to slip at varying degrees until 1800rpm. This is why companies like EA offer torque convertors that will slip at higher rpms for people running drag application 850's. As long as your torque convertor is working properly, and you don't do the brake-torque launch over 1800rpm too often, I wouldn't worry about it.

Certainly it's not too good for the tranny, though every now and then it's hard to resist;-)

Basically it depends some on the conditions, but assuming that you have a dry, grippy pavement and relatively grippy tires, I usually hold the tach around 1800-2000 rpms before I launch. Launching can be a bit tricky though, I'm still trying to find the balance between just releasing the brake and mashing the gas & too much wheelspin vs. feathering the gas a bit as you release the brake & too little power. In other words, for my car at least, it probably is not optimal to just pop off the brake and hammer the gas to the floor--just leads to too much wheelspin as the low-profile tires can't really handle ~275hp all at once. Practice at this and you'll figure out what's best for your car.

Cannot be good for the tranny - especially one that is noted as a weak link.

Just IMHO.