.... Currently I am happy with the OS valves and open exhaust and I will never take that any further.
When I compare the stock valves and the 1mm over-sized valves in our current rebuilt engine specs (261C cam), my DD shows only 1 lb. average increase in torque, and a peak increase of 5 HP around 5,400 RPM with the over-sized valves. That increase is about 3.1%. So from those numbers, the increase in power is at the top end of the RPM range, and doesn't seem to help that much for torque at the lower RPMs.
SEE ATTACHED EXCEL SHEET.
Compare the numbers from 1000 RPM to where the over-sized valves start to show increase.
The stock valves produce more torque up to 2700 RPMs.
Again... just numbers in a computer program.