as i said before, good luck with that.
you can not have the best of both worlds with a single speed bike.
if that were the case then the transmission would have never been invented.
if you want low end torque and high top speed, a shift kit is the only way to accomplish that (also knows ans a transmission).
you can, however, lower the gear ratio at the pedals by either putting on a bigger rear sprocket or a smaller front sprocket on a single speed bike.
40-19 gearing for peddling is very easy to pedal, and will give you low end torque for climbing hills.
a pipe will not help you gain more speed really (well, maybe 1-2 mph)
a pipe will give your engine more torque, and a little more power, but don't expect a 2 horsepower gain by adding a pipe.
the right expansion chamber can be tuned for maximum power at either the low end, mid range or high rpm's of the engine.