I run mine on 5s as well w/o problems and its the same rpm as the 2200 on 6s. Change the voltage limit of the 2650 to 5s. That would make more sense - unless there is something you are not telling us (why a 1515 size motor would handle more rpm)???
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy