XRT sleds go as well as the sedans .. a little heavier and rigid rear but otherwise just the same ...
Ford
PR might be the best avenue ... FORD have had bad experience with some of their cars being way under spec (eg most of the 99 Cobras in the US) ....
You have all the information you need by the sounds of things ... sounds to me like it's the ECU/boost controller ... has the ECU been replaced (not just checked but replaced) ?
Just go for the juggler with them .... 150 rwkw standard or 165 shootout is about 10-15% below par and the torque should come in clean @2000
Good luck and let us know how you go ....