I beat the hell out of my 4l60s and the 4l65 in my denali is smooth as butter. The issue with these transmissions is the lack of planetaries and the abundance of aluminum casting which is common to fail over time or heavy loads (or lack of lubrication). I'd take a 2500 suburban with a 6.0 any day, the 4L80 will hold way more power then these crappy little aluminum 60s. Built trans will set you back $2k or so plus a converter, there is a company now producing a stronger planetary gearset for the 4L60E/4L65E:
PATC has developed a 6-pinion planet gear set for 700R4 / 4L60E transmissions. This will be the biggest thing in 700R4 advancements in years. These will have a 2.66 to 1 first gear, 1.55 to 1 second gear, 1 to 1 third gear and .64 to 1 overdrive ratio.
This is a 36% overdrive making this one of the best overdrive ratios around. The 1st to 2nd RPM drop between gears would be slightly less than the RPM drop on a 200-4R transmission. This is what people have been asking for, for years. These will work on any 4L60E type transmission without a gear ratio error code, if the PCM is modified. This gear set is used to reduce torque multiplication / rotating speed in the transmission in first gear. This improves track applications that overpower the rear tires upon launch. This is common with numerically high final drive ratios. (IE 3.06). Now you will have a choice between the wide ratio or the new close ratio gear sets.