I have the Radion G4s and the RMS.
If I follow your suggestion, it sounds like you want to inset a spacer between the vertical arm and the horizontal rail? I think that would work ... there is just a single bolt on each side, so you'd have to replace that bolt with something longer and then put your spacer in between. Sounds viable.
Having said that, are you really sure you want to raise them?
BRS did a really interesting video investigation of the optimal mounting height for the Radion and they came to the conclusion than the 9" recommended height was actually ideal.