I found this site:
http://durangofansite.com/gearing.html
From looking through I think if we leave the 81 spur and changed the 29 pinion down to a 25 or 24 we should be quite close with the 9.5T motor, but I assume these are just starting points though as they wouldn't match up with the standard gearing on the rtr... Unless I am reading it wrong!
Have any of you guys that swapped to the 9.5 had a play with gearing yet?