You will see that the 3.0 has a larger bore and stroke than the 2.5. I don't think you can bore out the 2.5 the 0.24 needed, as most overhaul bores are only .060 or less if my memory serves me right, and I have not heard of very many BMW 6s ever needing the bottom end rebuilt until 200K plus miles. You would probably need a new crank and rods. It would be much cheaper and easier to buy a 330i engine. I think a few members here have done or discussed swaps of short, long and entire 330 engines into a 325, as well as ZHP and M3 swaps, which get more complex.
Correction, I read the chart wrong. The M54B25 and M54B30 have the same bore. I presume you could bore it out a little, but looks like a new crank could stroke it to 3.0. You are still missing the advance intake and cams of the 3.0, so still easier to get the engine.
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