I've run an IVO on a Lyc, for a very short time. I won't do it again. The ones I've seen (about 15 years ago) did not have drive lugs for the blades. Early versions had smooth faced hubs; later ones had knurled faces on the hub plates in an attempt to keep the blades from moving in the hub. Nothing was successful, and he finally quit selling them for Lycs. Sad thing is, they'd probably have worked ok if he'd designed them with drive lugs like all the Lyc props use. They seem to work ok on small 2stroke engines in ultralite-like a/c. The picture *might* get better on a rotary with its much higher frequency of impulses and no torque reversals like the Lyc. The other issue is the aero 'wall' at around 150-160 mph because the IVO blades don't have enough twist for high speed efficiency.
On the ratio issue, what HP is the airframe designed for, and how much fuel will it carry, and how much can you afford to feed the engine? With a 2.85:1 drive, a 300+ HP 20B is going to go through quite a bit of fuel. Even if you back down to 200 HP at cruise, that will be at least 16-17 gallons per hour of fuel consumption.
On the flip side, how big a prop can you swing on your airframe? If you use the 2.85, limit engine rpm to the lower figure, and can swing a large diameter prop (80" range), you'll get a huge boost in takeoff/climb performance with little to no top speed penalty, even with a fixed pitch prop.
FWIW,
Charlie