They do test them, but only against stock programming. Most of the chips call for more fuel (most tuners don't, they get the increase HP by advanced timing). My old HPOP would hold 2500 PSI with a Hypertech III, but droped to 1600 with the DP Tuner. By the way, the DP Tuner is WAY, WAY safer as the timing is much less than the tuners.
It is a gamble at best. If you get one from Ford, they don't replace internals except broken parts, and if the doner truck was not maintained (regular oil changes) then the pistons will have additional clearances, ie, less output. That is why I went with the Delphi. I talked to many vendors trying to get some kind of warranty, but as soon as you mention aftermarket tuning and a way to collect real data, they all backed off. I felt this was the best gamble next to dual pumps.