RE: Propeller acts as brake?
I think the speed record has been set by Dynamic soaring akin to slope soaring. I have seen these going around 300mph.
One note, I am instructing a friend with this Nextstar aircraft and it did not fly well until I removed the wing droops and I never installed the brakes in the first place. Also, IMO the prop that comes with the plane is junk. I do like running low pitch but I think a standard APC 11-5 would work better. I put a 10-6 on his plane and it works perfectly.
Also speaking tuning engines, the needle restrictor on this engine, needs to be removed. I turned the needle in 1 full turn beyond where the restrictor would have it stop. It is impossible to have a well flying plane if the engine is not tuned properly. It is just as important not to be too rich as it is too lean. An engine will never idle and advance properly if too rich.