The only problem with the PS/2 Keyboard route is that it is limits who can utilize their own hardware. Sure the organizer can have a few modified keyboard arcade sticks, but that does not solve the issue of allowing players to use their own stick/pad and in essence, forces a particular controller on the players. This method is also what limited the supergun route in terms of flexibility, until Undamned changed everything by releasing his consolised CPS2 setup and usb jamma adapters.
Regarding USB input lag…has it been 100% confirmed that all arcade usb sticks actually poll at 125 hz (8ms) on a pc? I did the USB poll rate hack (which for some reason only recognizes my MadCatz SFxTekken stick, but not my modded sticks via PS360 chip) and I didn’t notice any difference between playing at 1000hz (1ms) or the default 125hz. Which makes me wonder if either:
A) all arcade sticks poll at 1000hz
B) the USB poll rate program didn’t actually register the stick