I am not sure whether he was perfect casting or not.
I couldn't say that he was miscast either..
My over-riding impression is of a professional actor who was offered a gig that he never expected to get. Finding himself unable to turn it down - although his initial instinct was that he felt too young for it,
Having committed himself to it, he approached it totally professionally and did his utmost to "find" his Doctor against all the odds of poor scripts, producer impositions (hello celery) and a designed costume.
If he had not had such a spectacular finale in Androzani, and had regenerated during a humdrum story, then there might, I suggest, be a lot more ambiguity towards his reign.
But he went out with a bang instead.
I also think he jumped ship at the perfect time. It was left to Colin to drown on the ship and Sylvester to sink beneath the waves with it..
That could have been Peter's fate in the show, but he got out just in time i.m.o. and is all the better remembered for it.
A professional actor who did a professional job.
Despite falling short of the first four Doctors in my opinion, he still gets a love vote from me for his sheer professionalism and approach to the role.