Is it running the standard ecu? I had a car that was Sabre tuned to an alleged 400 bhp. I had it on my friends rollers and it wouldn’t make 300. It was over-fuelling and ran like a sack of shit.
I would be looking at the fuel pump and pressure, if it had all the mods removed and then re-fitted the pump might have been overlooked. It sounds like it ran very lean. Was it running direct and bypassing the boost controller?
There are so many things that could have caused the problem.
Who is rebuilding the engine for you?