This is because the local "transmission shop" only knows how to do a stock build with a shift kit. You start talking about a 500 hp diesel transmission and they are immediately out of their comfort zone. If you're looking at running that power level (and why wouldn't you?), you're going to need a well built transmission. A stock one with a shift kit won't get you there.I’m gathering parts to do a mild upgrade on mine now..
One of the things I’ve noticed after talking to two trans shops is that they both pushed me towards a stock rebuild with a shift kit and didn’t want to talk about anything else. “It’s a waste of money, you don’t want that” was what I heard about billet input shafts, upgrading the strut, lever, servos, clutches, etc. Even when I pointed out what I wanted to do HP wise (450-500 is my goal) they just tuned out and said something about warranty and never having problems with the 48re. My take away was that I hadn’t found the right place yet.
That’s a long way to say be sure you find the right guy!