have you tried a taller rear sprocket? i know you said your bikes idles at 6mph with current 12/51 gearing, i've always setup my bikes for 3mph at idle to go fast in tight gnarly single track, i think a 12/55T or 12/56T will get you there. Laugh all you want, it works. 2nd and 3rd gear really usable on tough climbs. The bike has 5 gears, why not use them ? Its not like the MX bikes are out running me on top end.