yeah, they'll hold 500 no problem. If the car is heavy, and a transbrake, it's quite likely to need a freshen up about once a year or so, depending on how much it's run. That is the problem with C4's. Seems like everyone I know who has run them (PA, A1, all the good builders, even my JY built home-brewed stuff) would have to go through the box every so often as the 2-3 shift would get sloppy after a while, or in some cases just lose forward gears altogether.
Another problem with the C4 is the forward clutch pack. As most of us know, the C4's clutches (fwd and hi/rev) are smaller diameter and they just don't have a lot of surface area. The cure to this is either (1) more surface area-which means more clutches, or (2) more line pressure. #1 has limitation because you can only fit so many clutch and steel plates into a C4 drum by it's design, and you can't go any larger. We used AOD direct clutch plates in the past, because the AOD plates had a very slightly larger friction area. #2 is not a good idea for 2 reasons. One, the fwd clutch pack piston return spring sometimes breaks with elevated line pressure (>150 psi, in my experience), and two, the valve body typically boosts line pressure-and increasing overall line pressure increases the reverse boost pressure. I've seen 350 psi in race builds. And I have also seen servos blown out of the case, almost always happens with a cold transmission and a driver not letting it warm up before throwing it in gear. Most valve bodies don't address this issue, some do now but not all of them.
The valve body is something that I have not messed with enough...so I can't say which builders do what. I always built my own late model VB
's and done my own home-brew full manual deal. My own experience with C4's has been exactly what others have had. Freshen them up fairly often and they work well (in performance oriented applications). In 2005-ish (IIRC) I build my first powerglide and tried it out. I haven't messed with a C4 since and see no reason to unless it's a period-correct restoration or something similar should I find a project of that sort. NOTICE that I didn't say that a C4 is junk. A C4 is a great transmission. But it's a small transmission, and thus when you start making some power, it could cost a lot more to build it and maintain it than maybe another transmission would. It would be a consideration if you're looking for longevity and if you're going to drive it hard often. Some guys will have 1500 hp and just cruise around with it, with a very occasional blast to boil the tires and that's about it. Those could easily last a long time. Then there's guys like me who drag race all year long with only about 45 days for freshen up. I typically put 200 runs a season, give or take. I went through plenty of C4's, and had extras built so I could just swap them in when the other was worn. When I first got to making some power and running a lot, I had 2 spares that went to the track. I would sometimes use both spares, and then rebuild one between rounds sometimes. That was before I figured out what I was doing wrong. After, it was one a season. I could get 2 seasons out of them but they would get inconsistent, specifically the 2-3 shift. Thought about a TH400 because I liked the 3 speeds but it, too is expensive, and heavy and power hungry. Thought about it more and tried a glide and for what I do, it's fine.