Hi Steve,
I can't claim any expertise, but I do have some experience with various prop diameters on RVs. Van started out recommending 68" dia props on the 2 seat RVs. My 1st RV4 had a Warnke 72x72. With 160 HP, it had 1200-1500fpm climb and would cruise at 170 kts @ ~9.3 gph rich of peak, ~75% power for a Lyc 160. The cruise matched Van's numbers, and climb was at least as good, if not better than Van's numbers. After my partner in the RV damaged the Warnke, I replaced it with a 68" from another mfgr and the plane never performed as well in either cruise or climb. At one point, I tried a 76" dia IVO. Climb was incredible, but the plane would hit a wall at about 145-150 kts (known problem with IVO's blades; they don't have enough twist for high speed operation). Another data point: Van's current catalog lists a Hartzel 74" dia c/s prop for the 2 seat a/c and the f/p props are in the 72" dia range.
My calculations on optimum dia & ratio for the Renesis are similar to yours. If you start with the Lyc-standard 2700 prop rpm, with the realization that 74" is not too large a diameter (for tip speed issues) at 2700, then the optimum ratio would be determined by your desired max engine rpm. With a controllable prop, you can give up a little bit in diameter & get back low speed thrust by going to flatter pitch & more HP. But if fixed pitch, I try for the largest dia I can safely fit on the plane. With a typical fast 2 seat homebuilt, gear leg length, not tip speed, will almost always be the limiting factor even at 2700+ rpm. The bigger the diameter, the better the low speed mass flow (thrust), and there will be little to no penalty at any cruise speed under the 180-190 kt range (well above 200 mph).
The above assumes a properly designed prop, of course. One reason to desire that 2.4-2.5:1 ratio is that it gets prop rpm where the prop carver is used to working, and we're much more likely to get a usable prop on 1st try if he's in familiar territory. If you tell him, say, 200 HP @ 2700 prop rpm & cruise at 75% will be 170 kts, he'll likely be able to get it right, 1st try. If, on the other hand, you're telling him the same HP & speed, but you want, for instance, 76" dia & 2300 prop rpm, he's going to be guessing on how to carve the prop.
Bottom line: I fit the largest dia I can safely swing, regardless of f/p or c/s (because low speed thrust will always improve with diameter increases), knowing that ground clearance will be a problem long before tip speed becomes an issue. If you haven't already bought the 72", I'd be asking them for a 76" if they'll sell you one and you can safely swing it on your plane. And be sure to tell them the actual rpm range where it will operate. Their 'stock' 72" for a Lyc will be sub-optimal turning in the ~2000 rpm range with a 2.85 drive, unless you intend to really flog the engine hard all the time & pour a lot of fuel through it.
FWIW,
Charlie