if wear isn't a problem, and neither is reliability, the sky is sorta the limit.
you can get custom cams ground to support about any powerband location you want, and you can find ever-stiffer valve springs to keep the valves where they should be.
lightweight pistons and rods can be made... proper clearances can be setup (to take into account the massive rod stretch at stupid-high rpms), better r/s ratios packaged, etc etc.
non-pneumatic valvetrains usually have a max of, what, like 12k rpm (whatever you see motorcycles redline at). that's where spring pressures get too high and start shrooming valves.
then you get to find tranny gears to use a stupid redline like that. the whole things kinda pointless, over 9k, or whatever typical off-the-shelf cams like.