You know, don't know if this has been fixed or not (or whether it's actually an issue for most), but early on when GT5 had just been released, I found online breaking in not to be as 'reliable' as breaking in a car offline, sure the mileage went up but it didn't have the similar effect.
Meaning that some cars which had done significantly more miles than the required 300km/180miles for a full break in, still hadn't achieved their peak power (which they eventually did after doing plenty of miles more).
As a result I break my cars in offline first before going online with them ever since noticing this discrepancy occurring sometimes (not all times though).