A stock GM built tranny with a properly installed Transgo kit is one of the most durable 4L60E's you will find.
In my Impala: lasted 13 years with hundreds and hundreds of motor passes all the way into the low 12 sec range, and probably 75 passes with the 150 shot (425 rwhp) down into the low 11 sec range. I finally broke a hard part in it with a nitrous launch on a poor starting line and about a 90 mph 3rd gear re-hook.
In my pickup: 298,000 miles of "spirited" driving, towing, and 1/4 mile passes, sometimes with the 50 shot, running high 13 sec passes at 4950 lbs. Tranny still functioned perfectly in everyday driving, but nitrous passes at the strip exposed 3-4 clutchpack wear.
I wouldn't expect to get that kind of longevity out of anybody's rebuild. Don't get me wrong, if you're really pushing some torque and rpm through it, then there are just a handful of rebuilders in the country that can help you. But if you're stock + 50% power level, GM gets my nod.