Pusher Propeller Spacer

I have a RANS S-12 with a 912UL on it and a 75 inch 2 blade PowerFin. The Fin is on a 55mm spacer. It flies fine and cruises at 80 mph with power settings 4950-5100 depending on the atmosphere. 

My issue is noise. Even with earplugs and Lightspeed ANRs it is ear splitting. I have tried different exhaust solutions and have settled on a CKT that seems to have muffled the exhaust as good as I can get  it with the engine being on top of the wing.

I’ve investigated a lot of props and will be trying a 67” 3 blade e-Prop from France.

Besides the noise from the high tip speed of my current 75” 2 blade, all pusher props interact adversely with the unequal speed airflow off the trailing edge of the wing. I’m thinking that -both- lowering the tip speed and changing the “tone” by adding a blade will help ameliorate the noise problem.

Another method thought to reduce pusher prop noise is to move the prop back from the wing. e-Prop claims I can use a spacer up to 170 mm “without risk to the gearbox”.

Here’s the question: Does anyone have experience or experiments that would indicate the potential noise abatement effectiveness of, say, a 20mm spacer vs. a 170mm spacer? I’m interested in “real world” experience - not theoretical stuff. 

Also, I’m interested if someone with knowledge regarding mass dynamics agrees with the factory claim for the 170mm spacer. It is listed as 255 grams.

Thanks.