I've just measured my stock A&K SVD barrel for you:
ID = 6.08mm
OD = 8.51mm
The thing about R-hop is this, the hop set up you are already running can lift the heaviest BB you intend to shoot so much that it over-hops, ie rises in a daft way, then you cannot gain anything, can you?
I haven't done as much experimentation with mine as i should tbh, but i suspect that i can get the airsoftpro.cz advanced hop unit (the same one ASPUK sell) to overhop 0.4's with the right rubber... and that is not complex to use nor is it subject to being knocked out of alignment.