Well, I actually am only having Aamco install a rebuilt tranny, not rebuild it. Also, being as the Aamco guy tears down ones that have failed and head mechanic tested some but didnt see them after 20 or 30 k miles or more hard to say how he could say with certainty. Also Mercon III is semi synthetic, V is full synthetic. Not exactly your conventional oil to synthetic arguement.
Only reason for swapping my tranny is because car is 14 years old and seals were failing. I figured while it was out , may as well rebuild it. It had 100k miles on it and still shifted fine. It may be prone to problems with guys pumping up there motors far beyond what Ford had intended for the tranny's to deal with. However, I am running stock and intend to keep it that way for a daily driver. If the 4R70W were meant for 300hp+ then it would have been beefed up similar to the later model DOHC mustangs versions. Anyhow to each there own.
As someone who also fools to much with building computers, I'll tell you the more you push something past what it was intended for can get great initial results but for how long? Without a big re-do and beef up to the internals(tranny, remachined head, etc), its no wonder guys blow so many head gaskets and ruin tranny's. Trying to do build ups on the cheap only causes more trouble then its worth. Precisely why I am not fooling with mine. I have not the cash to do it right from the top down, so there is no point to try and squeeze a lil' more out of it. No good could come from it.
/rant off.