His plane is already a dog with a 8x6, moving up to a 9x6 will only make it worse. I've run enough .25s to know that a 9x6 is an overload.
Are you saying that you have already tached his engine with a 10x3 and clocked the plane? Even if you haven't, 35 mph would be a doable cruise speed for any normally loaded model. I will bet that 40 mph is a more realistic actual speed.
I've spent 90% of my ".25 time" using 9x4 props on typical sport models and have gone up and down the ladder in the past looking for a better working prop, but always return to the 9x4.