yup. i'm running a 28t now. it's a little slower (very little) off the line, but i also have 24" wheels, so there's less rolling resistance and quicker acceleration compared to a 26".
i have a 26" bike with a 34t, and it takes off much slower, and takes awhile to get up to top speed.
there is a lot of other factors involved, though. the 26 is heavier, and it has a basically stock motor that hasn't had all the crazy work done to it like my 24. it's also a Grubee GT5 Skyhawk, while my 24 has a GT4 Starfire. personally, i think the GT4, which according to Grubee has slightly less horsepower than the GT5, is a stronger motor.