Well fuel dispersion is pretty well handled by the injectors and combustion chamber swirl. Nothing much is going to happen in 1.5" (est) of the port. That why top Honda tuners reshaped the combustion chamber and not the ports.

My point is airflow is crucial and the only obvious impediments are port matching and the huge (relatively) valve guide boss.

Regarding valve spring rates: It's a compromise between valve float at high RPM and seat pounding at lower RPM.