I guess I'm getting in on this one a little late but I have to think the topic of this thread has gone off what the original poster asked. He wanted a faster roll rate. To me that means angular speed (degrees/second improvement). Aileron differential is great for improving tracking in turns but I don't think it will improve roll rate much. In fact I lean toward SportPilots opinion that it may even reduce rate.

One thing thats fun to do is take a large wing out in the backyard and twist it around in front of you like a batton to simulate the rolling wing on an airplane. You'll see that as you try to increase the angular speed the air resistence builds up rapidly. It's because the wing's plan area must displace a very large amount of air. Biplanes and delta's tend to have a much higher roll rate because the wing is not having to dig that big screwthread through the atmosphere.

After checking for underperforming servo and properly sealed aileron gaps, the only way I can see to dramatically increase roll rate is to increase aileron area (displacement angle from neutral) or shorten the wing. Fifteen years is a long time and it's likely there have been better planes flown since then to increase the OP's expectations. I loved the game Fallout back in 1998. I installed it recently for a playthrough on the PC and couldn't believe what a piece of crap it was!