You will probably need a larger output sprocket on the transmission to get the speeds you want. Last year, we used the AM single speeds (same ratio as the ToughBoxes) and had to go up to a 21T sprocket from Small Parts on the output to get to ~8ft/s. To get to 15ft/s you would actually need to gear up from the transmission output to the wheel. Something like a 1.73:1 gear up should work for that.
1.73:1 is a decently sized gear up (well, at least for going from the tranny to wheels), so you may want to consider going to larger wheels. If you want to stick with 6" wheels, an aluminum output sprocket is probably in order (a steel one large enough to get that gear-up would be really heavy); unfortunately, aluminum sprockets with 1/2" bores and 1/8" keyways are hard to find (assuming you'd rather not broach your own keyway). To that end I'll leave you with links to two parts that can solve that problem, but how to do it is left as an exercise for the reader (it's not that difficult, you just have to think creatively)
Link 1 Link 2.