With some brands of radios (particularly those using direct sequencing), I have experienced glitching (range issues) when the Tx voltage dropped when using 6S NiMh cells. After switching over to 2S LiPo, this boosted my voltage from 6V to 8.4V and this eliminated my range issues on large 1/8 tracks. I wouldn't doubt there is a point of diminishing return where increasing voltage can only help so much, but it would be impossible to say for sure without testing. I don't see any reason why it wouldn't be worth trying to boost the voltage on the Rx provided it is rated to handle the power.

Good Luck!