I agree that you should try a 9x4 and 9x5 prop sizes.

The OP said "old OS MAX .25", which may or may not be a .25FX. In fact, it probably isn't an FX. Like some of the other guys here, I've run a lot of .25 size engines over the last 20-odd years, and a 9x6 works ok on light, fast airframes, but I think in this case a smaller prop is the way to go.

Also keep in mind that these engines aren't linear. Somes you actually pick up airspeed when you go to a smaller prop. I've seen planes fly faster on a 9x4 than on a 9x5, and the 9x5 was faster than the 9x6. The reason is that the engine makes so much extra RPM that it more than makes up for the drop in pitch, while the airframe's drag prevents the engine from really unloading with the 9x6.

You might try a 10x3 as welll, but honestly, that prop was picked for LimB and 2548 combat to keep the engine performance down. If you put a 9x4 on the same airframe/engine, you get better climb and speed and a lot more RPM.