I have 350 hrs on my IO-320 RV-8 with a 2 blade Catto at 1025 lbs empty and it performs quite well. A few key items to consider -
An IO-320 powered RV-8 will perform as well as or better than most of the other RV-Xs with a 320 of similar weight and wing loading / CG. The narrow fuselage does help in lowering drag. Depending on W&B, it will likely perform as well as or better than most of the wide body RVs with 360s and FP props. It will not likely out perform most RV-Xs with a 360 and a CS due to basic physics. Consider handing qualities, a light well balanced RV-8 is much more fun to fly acro than a nose heavy RV-8...which you can always ballast the nose heaviness, but that increases wing loading.
My IO-320D2A is a "parts motor built by Aero Sport. It could have been a carb'ed or injected motor, but I put a vertically mounted FM-100 on it and it became an IO instead of an O. I wouldn't get hung up if the IO-320 was a parts motor and not from a pedigreed certificated family.
I have done two up acro (spins, aileron / barrel rolls, loops) with no issues, it all depends on the empty weight and learning to the aircraft, not fly the engine and prop.
Are you buying this acft as an investment or to fly it and enjoy the gift of flight? I wouldn't worry about re-sale, there will be an informed buyer out there who will be glad to buy what you have.
If an IO-360 with CS prop was delivered today, would I put it on? Yes, absolutely, but until that happens, I will enjoy the fact that I have a safe, fun RV-8.