Not 100% sure what the threshold is, but I know they stand up to boost fairly well. They've got decent pistons and forged connecting rods. Much tougher than the old 3.8L and certainly more stoute than the 4.6L. I've seen/heard of guys putting 350-400hp through them with little fuss. The biggest problem I've ever heard of with the 4.0L SOHC is just the odd cracked exhaust manifolc (trucks) and the common noisy timing chain issue.
rygen had a 10 second 4.0L stang. the connecting rods are the downfall. even aftermarket offerings are not that good. a custom set of rods are needed for serious power, to the tune of about 3 grand per set.