I also have a mtb that is too small. I did replace the seatpost with a longer one and it made enough of a difference for me to start commuting, but I too hunger for a faster bike. So in my experience, a longer seatpost may help you put off the purchase because it does make a big difference to have your legs in the right place. I've been sorely tempted to buy this http://www.bikesdirect.com/products/...e_xi_steel.htm if you can put together the $300 and don't mind buying online/not from a LBS/putting it together yourself.