I experienced the same effect as rshubert testing OPTEST4 online.
Not sure if I was missing most of the time or most hit flashes are gone in this version. I was aiming and firing as always but with far less vissible hits and inflicted damage. In some cases I got kills with no vissible flashes at all, while debris and fragments where clearly vissible.
I tested OPTEST4 online with VSYNC ON, Mindanao Map, all the time firing 190s guns and mgs.
BTW, somewhat I managed to keep the system using only 270MB (Win98SE here), this way I got an average 25% more FPSs (OPTEST op objects and op terrain ON) than normal AH.
On my last flight, during a low level dogfight I kept sending check six messages to friendly planes in front of me. To my surprise, the messages where all the time sent to players not visible nearby. I suppose these players were in GVs near and in front of my plane, but their icons where not visible for me.