Seat wise, I have to think there's a difference in spec between a street engine vs race. I've seen some big valve race heads with really close seats/valves. How much heat was transferred from the exhaust to the intake, I don't know. After all, they still occupy the same space. But, from a durability standpoint, especially on a street or a endurance engine, I have to think more clearance is better. I will be watching for other opinions.... Never to late to learn. :D
On the bore clearance, my thoughts are, as long as a larger diameter valve doesn't get to the point of wall interference, the little lost against the bore, will be made up for, or more, in the rest of the circumference as far as breathing is concerned. Is that correct? I'm not totally sure, but my thinking say's yes.....in my mind...lol.