G'day Presso,
If you use more than 5% nitro, the A3 is too hot, & it is a plated plug anyway, so it won't last quite as long, don't be afraid of the #8, it works fine in all my 2 strokes from an OS 25FSR, 2 x 46AX's, a 61FX, 75AX & a 160FX.
I use 10% nitro, & OS #8 plugs in all of them.
Carbon fibre props are rigid, in other words they don't flex like Master Airscrew, but are very expensive, APC are efficient, by being stiff, not as stiff as CF, but not as expensive either.