Sounds like your 2.5 needs a tune-up. I bet you're not pushing more than 7-9lbs boost, if that. Should be 13lbs. The 2.5s tend to lose boost w/ age. Btw worn wastegate spring, failing vac moduals, over-protective over-boost systems, they tend to lose boost fast. Check the boost and see what you find.
I'd never trust anyone who puts and old motor into a newer car. If they're trying that hard to save a buck who knows what other corners they cut...
...Like spray painting an entire long block silver so it looks new...