I have a samsung 226bw 2ms and if my understanding is correct, the rta mode is the overdrives and when the rta mode is turned off, it performs like the 225bw 5ms display. If i'm understanding this correctly i'd say i can definitely tell the difference between 2ms/5ms when rta is on / off.
Remember the 2ms and 5ms is grey to grey...the 2ms ones use overdrives which mean in black to white the 2ms perform nearer 10-15ms in extreme cases whereas the 5ms+ (ie no overdrives) can perform waywords of 20ms + black>white.
Bottom line - The overdrives in the supposed 2ms panels helps lots in fast action packed games.