Even with the non-VVT issues
The B5254(T2), B5254(T2-R (or B5254T4)), B5254(T3), B5254(T7) are all VVT engines. There shouldn't be an issue there. He could probably reuse the old camshafts and crankshaft if he wanted to.
totally different fuel maps?
There shouldn't be much of a difference of fuel maps either. Slightly, yes.
I run a 9.0:1 compression and it ran fine. I had the exhaust valves fail, but we think that was contributed to re-using the old sodium valves. We noticed in the Volvo Engine documents where it mentioned not to re-use the old valves. The ceramic shop sanded down the edges to clean off the ceramic so that it wouldn't cause sealing issues during lapping... and that added to the problem. Everything else looked fine and we are running again as of this week with the 9.0:1 compression. But overall, a slight compression ratio gain should be ok while breaking in a new engine. If he went with something higher than 9.0:1, yes, he would need to remap it right out of the gate.
The higher compression will actual boost faster, not later, since it's a hotter compression and also the additional power before the turbo kicks in.
I'm also assuming all 5 cylinder engines fire off in the same order... per a harmonic design.
Why do you think it's the cat?
I've seen in a couple cars where the EGT was raised and it destroyed the CAT. The symptoms were later spooling because the clog prevented flow of the air in the turbo to spool. He may also have had a CAT problems before hand when he ran out of radiator fluid.
Another thing to consider is that engine shops and car manufactures will typically throw in a higher phosphate oil to help the break in process. I used royal purple myself and that has a special formulated phosphate blend. One of the things phosphate does is destroy the CAT. If he's already experiencing a symptom that may point to the CAT, he may have just increased a chance of destroying the turbo and exhaust valves right out of the gate. To check, you just have to remove the 2 bolts on the exhaust / DP connecting point and peak in.
Also, I would believe the CEL would come on for an open wastegate problem because the MAF and Throttle position would be off from what the Boost Sensor shows... just a guess though. I believe a boost leak would show a CEL too.