fixy bikes have only 2 speeds.... fast and even faster lol
they are very light and very nimble and even with a stock 44 the light wieght of the bike and the larger diamiter tires and thinner tread pattern and low rolling resistance, make it quicker than the standard 26in bike by far !
now a stripped down fixy bike with a 36 t rear good god man !!! thats a 45 mph bike lol
the only problems i can forsee is the lack of suspention .... you give up the cushy suspention for a fast ridgid nimble frame, you go fast and it rides sweet but you feel every little bump. and mabey a lack of breaking power, i know some fixy bikes only have a rear break side pull style, you want to make shure you have front and rear breaks and good ones too help you stop !