Regarding tune, just a theoretical WAG (I would encourage you to start your tune over again) but I'd guess the low speed wouldn't change much, while the high speed could go either way - but not much. Maybe a screwdriver blade width.

Your "prop rule" is one I haven't seen. It sounds close - but only as regarding engine load. When you change the propeller's pitch there will be an associated change in the planes performance envelope. 6" props will generally result in better braking, lower top speeds, better vertical, and shorter take off runs. If you know the plane well, all of those attributes will change with an 8" prop - faster top speed, less available braking on down lines and while on final, etc. Basically, going to an 8" prop is like shifting the plane into a higher gear if that makes any sense.

I know they don't give away props, but you really don't know which you'll like better until you try them (6" vs. 8"). Another WAG - 6" will usually work out best for most sport flyers....