I hadn't heard of Steve or Commercial Powertrain until I read this thread, but my point--aimed at the OP--is that these aftermarket "built" transmissions are very, very complicated pieces of machinery and are not a plug-and-play modification. I have been spoiled working on my Cummins; a lot of mods that make a big performance change are relatively easy to install and require ZERO development on the part of the consumer.
68RFE's are complex, and you will hear a dozen different opinions about the best way to make one hold together under extra power if you talk to a dozen different trans guys or speed parts shops. If you're coming into it based on a little forum research, it sounds as though installing a built trans is as simple as a new turbo or head studs, but getting it to work right is serious business. I believe this is why so many guys with heavily modified trucks are running older, simpler, 4-speed autos or converting to manuals.
Guys swap to a 47 or 48 (4-speed) for faster et times at the track. As you can see 1st is taller, 1:1 is reached sooner, and o/d is a little lower. And not unlocking and locking the converter for 2 extra gears. And we'll just leave out all the tuning stuff with the PCS computer to run it..
Aisin as68/69rc (for comparison)
68's have two overdrives that are nicer for crusing down the Hwy(rpm lower for the sticklers out there) and a lower 1st gear better for towing. But the o/d clutches are also the first thing to go(stock trans) hard excelleration into 5th then to 6th. Have a look at the gear ratios and the difference between all 3 trans this should give a better idea of why the 68's are slower but not nessisarily weaker. And probably why you won't see any records being broken with a Aisin workhorse ripping down the track. (Someone frisky out there probably has BUT very rare...)
I'm running completely stock tcm tunning and doing just fine. Tcm commands 160psi, stick a gauge on the side of my trans and you'll find 225psi pretty damn quick. Get the right gear head to build you a trans and you won't have to worry about it. A CP trans could have been shipped to me under 1 condition: no after market tuning what so EVER. (This is what I have my fingers crossed for Steve to send them across the border to get things started) So saying is my trans complex? Nope, but it has bigger shafts and a few other goodies compared to most..
And the moment I change anything via trans tuning, well now the equation changes and other modifications need to be made to compensate for this.. So if you put it in and leave it learn, yeah it will take everything you throw at it, but keeping in mind which trans, 48 or 68 is best suited for your needs.