OK. So if it is bored over then it is no longer a 2.5L and I would expect a little increase in power. If the compression ratio is higher then I'd also expect a boost in peak power. I opened up my dyno2000 software and modeled a 1mm overbore 2.5L with 11.5:1 compression with all the most generous settings for airflow and exhaust, and the best I can get this to simulate is about 230 crank Horsepower. That might be about 185-193 wheel horespower on a dynojet assuming a 16-18% driveline loss.
That puts it online with the best of the 2.5L engines to date with just 10:1 compression ratios. Even if I assume just 14% driveline loss you it only comes out to around 195-196 wHP. EVERY mustang dyno I've seen has more like a 22%-24% loss. This would put that same engine around 174-179 horsepower at the wheels.
So I think this is BS or your dyno sheet was the wrong one.
On the other hand the dyno just could be out of calibration. If that was a legit graph that he gave you then you should get a comparison dyno done. Also, never do only one run on a dyno. Three pulls is a minimum to ensure that the first one isn't a fluke. You can average the closest two dynos to get a close idea of your actual power. Then you also have to disgard sudden torque spikes because that causes errors in the dyno reading. If your IMRC was fluttering and it caused surging with sudden torque spikes then the whole dyno isn't accurate anymore.
Still can't believe that a bored over 2.5L would pull that. If you just do a back calculation and assume only 20% loss then you are telling me that your 2.5L is producing roughly 265-275 crank horsepower.