I know this isn't really helpful but I've been fine on xbl 90% of the time. What are your speedtest.net results? Mine are ~1.5 mb down and .5 up. Try the 3g supercharger script if your rom has init.d support. From the tests I tried on my old cable internet having an open nat only really affected the time it took to find a game, it didn't really help with in game lag.
Do you have a good signal? Are you running a computer on the same internet connection? Check these things first. Even if you're not actively doing anything on another computer shut it down or disconnect it because the computer has to send keep alive packets to the phone/modem which could be slowing your connection (it does that with my girlfriend's laptop even if she's not using the internet.)