Fastsky: For pure speed my vote is for either an APC or MAS 10X5 for the 40FX and 10X6 for the 46FX. You can go faster if they were piped but putting on larger props like an 11X7 on the 46FX will not overload it on the planes you have mentioned however it will take longer to get up to terminal speed. You want higher RPMs in your case not more torque load on the engine although there is a trade off point to all this. Keep in mind that 40 size Q500 engines often use smaller than 9" props as they want max RPMs which gives them both acceleration and good top end.