Is the lower on a sportmaster just raw aluminum??

Yes, and is relatively a "rough" surface. Flat polished or painted surfaces chase water stiction which increases drag and causes turbulence in the water before it reaches the prop.

Many think a high gloss surface would reduce drag, but when dealing with hydrodynamics it is the opposite. Races teams are known to sand the hull surfaces that are in contact with the water.

So what if I just sand or strip lower to raw aluminum? I need to paint my outdrive anyway...

I don't know what speed the hydrodynamic effect takes place. The raw aluminum requires maintenance. That's why most mid performance boats have painted lowers

The boat never sets in the water for more then a week at a time in fresh water, so maintenance wouldn't be an issue.

Mine is never in the water more than two days fresh water and I find myself sanding the lowers with a scotch brute pad all year long. It's not a big deal.

Like I said, paint is convenient for most. And I don't know if it is better for a drive at any speed, I think higher speeds is where it comes in to play.

No harm in going bare, even if you don't benefit from it. I actually prefer the bare look on the lowers. I can tell you removing paint from a bravo drive is no easy task, it's on there!

I doubt you would be able to measure the difference. Maybe slight + speed at 100 MPH? You would probably get more speed if you took out the ice chest out or got a skinny girl friend....no nice a_ _ like the above avatar!