Well, It gets down to valve dwell. At some point your velocity will not increase any more, however your gas consumtion will (this would happen above safe velocity by quite a bit I would think) Idealy (more so for unregulated guns) you want some adjustment so that you can adjust the dwell to keep the valve open longer for cold days and such. You also dont want to stay open any longer than nessisary so you are getting the max efficiency. Now throw in hammer bounce. essentualy you are re opening the valve and you dont need to. Its like that thing (which name esscapes me) with the 5 little steel balls that swing. The motion gets transfered. First from your mianspring/hammer to the valve/valve spring, and then the valve closses and sends the hammer and mainspring back again, which then hit the valve again. A lighter hammer and heavy mainspring should help it so that the valve doesnt re open. However its all a balancing act.
An idea of mine has been to have a light hammer and heavy spring which is shortend so that there is play beween tha hammer and the valve when the mainspring is fully extended. That way energey is less prone to be tranfered through the hammer and back into the main spring upon valve closing. Im actualy bulding this set up in a sheridan based project. Im usign 1/16th inch of play. I dont know if it will work or not but it makes sense to me at least