I would not recommended it. Check my build
here. It's a custom XL length Savage but lighter than a stock Flux. The T8 1550kV(truggy) on 6S only just cut it with stock gear ratio. I would go a larger motor for a 1/5 build unless you are looking at really low gearing.
The RX8 was fine in my build but it's going to depend a lot on the weight of your build.