I didn't mean to imply "skipping" the underbody, I was just hoping to give some insight to the problem stated at the beginning of the thread in the most general terms possible, while giving some more helpful information if anyone was curious about certain aspects of the physics of just the rotating stuff.
Everything you mentioned was correct mcrews, however, we cannot say which is more or less beneficial without running numbers. If i had to guess, if you were running steel hubs, and switched to wire thin aluminium rims, you'd be able to feel the difference. The cost will be very large if you're buying new, or you could go to the junkyard and pick out about anything other than a steel hub and save several pounds, probably for a decent price too. After all, it doesn't really matter what it looks like if you cover it up with a nice aero wheel cover!
Some more evidence that the underbody is the MOST important part of any aero project: research in racing shows that ground effects can begin to produce usable downforce at as low as 20 mph, vs. 50-60 mph for a rear wing!!! This means that for around town cars, having an underbody is completely necessary for getting MAX mpg. In order to achieve decent downforce, the race cars must be very low to the ground. However, since we are not looking to produce downforce (drag), but aerodynamic efficiency, lowering the car is not necessary, but wouldn't hurt.
Hopefully I'm not preaching to the choir about this stuff. I've just spent my whole life learning about cars and want to help everyone achieve exactly what they want out of their cars.
As much as I love racing cars, I can't help but be completed enthralled by this website I stumbled upon this past week. I love seeing a positive community committed to learning what they can about their cars, and wrenchin their way to more efficiency and happy wallets.