I have been flowing and modifying and making heads for about 35 years now.
On the photo,s a cil. head for Honda 400 Hawk? I think. Made for dragracing in 1980/1981. And a head which I recently made for KTM racing with as you can see 4 inlet valves and two exhaust valves.
In every head design you have to choose the inlet channel so that at max. torque the inlet speed gets to mach 0.5 (approx 160 m/s at max pistonspeed position) for best performance. Honda made some publications on this subject with formulas added to calculate the best dimensions.
About the restrictions, you are right there. The main restriction is the inlet valve. Even at full lift this restriction is larger than the inlet track or carburettor.
Jan