Currently I run a 74/42 prop on my 0-235 in a Tcraft F21. What would be the advantage in speed and climb going to a 80/40 prop? The plane gets off the water quickly on Baumann 1500's but I could always use a little more cruise speed
More diameter is not conducive to improving cruise speed. Towards that end, you'd be better off adding an inch or two of pitch to your existing prop. I think the prop you're considering might not be a good choice for you. The pitch is kind of big for takeoff performance with a 235, and the diameter is going to hurt cruise.