I would recommend both a ratcheting harness and a dual lap belt. Sure, you're only doing Sportsman, which -- assuming you're not doing a separate freestyle -- is essentially all positive loading except for the momentary negatives on a couple of 45 degree lines (half cuban, sharks tooth, etc). But that assumes you fly all the figures correctly. What if you err on a figure? It's easy for unusual attitudes in aerobatic flying to end up with a) more loading that you expect, and b) negative loading, especially in a highly pitch sensitive aircraft like the RV8.
The ratchet is extremely useful for tightening the lap belt in flight. You can crank down the belts as tight as you think possible, and then after you've done a G check on base for your box entry, find after returning to upright flight that you can crank the belts down a lot further all of a sudden. The seat cushions, the seat pack portion of your chute, all that stuff compresses when you start pulling 4 or 5 G. The belts themselves even stretch.
When I flew the Pitts and Extra -- either with students or in competition -- I always tightened down the lap belts until it was almost painful. By the end of an intermediate or advanced sequence, they were usually measurably loose again. If I didn't have some bruising after a session of advanced practice, i wasn't tightening the belts enough. Most of those sequences were red, dashed lines.
Anyway, I think the dual belts are a huge boost for safety, and the ratchet just makes life much easier and more pleasant, regardless of what level you're flying. Just getting in and out of the belts is easier when you can rachet them closed and easily release the tension when you want to egress after a flight.
I've got a 5 point Crow in my RV-6 (single lap belt), and as nice as they are, I do miss the ratched and the security of the second lap belt when doing aerobatics.
--Ron
