Are you sure about being able to tailor the gearbox? There was a time when most gear box equiped motors were offered in a handfull of gear ratios. Last I checked 6.7:1 was all that was available. I do know at one time Neu and Hacker both offered 5.2:1 because I owned that ratio from both MFGs. The most flexable gearbox out there is no longer made. It was from MEC and they offered a wide range of pinions and 3 or 4 different spur gears. They were loud but they made your motors very flexable from one plane to the next..... These days motors are basically one trick ponies.... give or take a cell.
ORIGINAL: 2Sunny
A Plettenberg 30-9 on 12S would put you in that range:
3.7 volts nominal x 12S = 44v
44v x 235 kv = 10340 RPM theoretical
subtract 10% for estimated actual and you have 9500 RPM more or less and then I would say based on a 20x13 using 90 amp I'd ballpark a 20x20 at 120 amps which puts you over 5000 watts output. I'd bet the Pletty can handle that for a burst, but that'd be a heck of a load if it was constant.
On the flip side a Neu F3A would do the same and you could tailor a gearbox to your exact specifications.
Both motors are available at [link=http://www.f3aunlimited.com/webstore/index.php?main_page=index&cPath=9&zenid=h4n8u6qvi9 lcfu2up6edpgcn41]F3AUnlimited.com[/link]
Joe