I know this to be true as well, but in other speed contests, the guys at higher altitude say we have an advantage. (I also live near sea level) If this were true, submarines would go faster than airplanes and I'm pretty sure they don't!
I do know that amp draw will drop at higher altitude, so you may need to prop up or add another cell to get your watts back up.
Kyle, lots of homebuilt airplanes have been put together without a vacuum bag. Nothing wrong with that. If you want to block sand the final result no matter what, it can save time not using a vac and mylar, but it will be a little heavier. If you want to try a published airfoil in the future, you can make outside templates and just check the progress of your sanding at a couple spots. This sort of thing can help you reproduce the performance on the next one, even if it's just to reproduce a shape you came up with yourself.