Rossi (AXE) and NovaRossi (REX) are two different manufacturers. The NovaRossi will run just fine on 15% Nitro. I had the older NovaRossi C60F that ran extreamly well on 10%. Easy to start and tune. The NovaRossi (REX) CR46/57 are timed for 3D so I wouldn't expect them to work well in a speed application.
Rossi are out of box set up for 0-5% nitro. You can run more nitro with the addition of a head shim.
The Rossi (AXE) .53 5 Port or the Jett .50 would be my choice for all out speed in a sport plane. The Rossi will outperfom the Jett and do it on 5% Nitro. The Jett has the best carburator on the market.
NovaRossi (REX) engines are sold through www.planethobby.com
Rossi (AXE) engines are sold through www.rossienginesusa.com