Momus, it’s good to question the status quo of ‘good practice’ for sure. Especially from someone like you who brings in NASCAR thinking. There’s a lot old good old knowledge in the old bike world, but there’s a lot of old ‘folklore’ too!
The port and valve topic has actually been looked at quite scientifically on this very forum though.
I’d suggest reading the ‘cylinder head flow’ thread and the ‘time for spintron’ thread, both by Comnoz.
His findings show that big valves help with the port throat, and increase power and (contrary to most people’s presumptions) they increase mid range torque as well.
Like most things though, things have to be done as a ‘package’ to get given results.
Comnoz did my own head as a package and the results were astonishing (like a 9bhp increase and an increase everywhere from 3k upwards, measured before and after on the same dyno).
Both of the above threads are long, my advice would be to get in your best comfy arm chair and start from the beginning!