I'd guess that motor temps would be an issue when using 60c cells and trying to gear up the motor to retain the speed.
I'd work out the overall top RPM of the motors on 6.6v and 7.4, then get the equivelant motor.
6.5 LRP x11 = 6900kv = 51060 RPM @ 7.4v
51060 RPM / 6.6v = 7736kv
5.5 = 7100 kv
4.5 = 7800kv
So I'd be tempted to go for a slightly under geared 4.5 to replace a 6.5
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Altho I could be completely wrong
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