One thing to remember when considering tuning a RV8, is that unlike a modern engine, they dont age particularly well.
The camshafts and valvetrain tend to wear out which reduces engine power gradually over time without you really noticing.
So actually, the gains of sticking in (for instance) a upgraded camshaft are twofold, you get the original claimed 5 or 10% boost, but also regain the missing 10-20% due to the original camshaft having turned itself round. Now clearly most of the gains actually came from the replacement of worn out bits, but the costs of upgrading the cam when your replacing it anyway are fairly small, so you might as well.
The real issue is once you start, where do you stop. The valves will be leaky, so you'll want the heads off and a valve job done, and while your there you might as well port them, the compression on the cylinders will be iffy, and if your stripping the block to hone or re-ring, you will probably start considering top hat liners. You end up going down the route of a complete engine rebuild which you'll sink about 5 grand into.
Its something i've been struggling with on mine for a while. I dont particularly like the rover engine, and mines suffers all the above issues along with hopeless fuel consumption and a good thirst for oil as well. The idea of pouring 5 grand into it is something that i just couldnt deal with. I'd rather spend the 5 grand on an LS conversion or similar.