you can check the actual lag by moving the audio delay slider all the way to the left end. It will show you the actual lag you currently get. I think the Edge has got 8ms in gamemode.Lag is minimal via the EDGE when you use the GAMER MODE (6ms IIRC).
I would also advise to turn the Gamemode off for 480i when the type of game allows it. The deinterlacing without gamemode if breathtakingly good and at roughly 3 frames delay, it's still acceptable for anything RPG-like. Also set the deinterlacing to forced video to avoid deinterlacing errors caused by filmmode/videomode mismatch...
shouldn't be a problem. There are so many skilled modders around that someone should be able to help you out. For easiest handling, get a female Scart plug with 6 RCA plugs on the other end (R/G/B/S + Audio L/R).Maybe I could pay you or somebody else to put this together and send it my way?