with the added circumference...is there a formula to calculate diminished delivery to the wheels?

 

This C&D article may be helpful.....

http://www.caranddriver.com/features...d-tires-tested

 

I don't think its the circumference you should be concerned about. If the wheel/tire combo is properly sized (+1 concept) then the overall diameter (and hence the circumference) of the setup should end up close to the stock setup. The inch of diameter you gain in the wheel is offset by a corresponding decrease in the aspect ratio of the tire (shorter sidewall). What you may gain is increased unsprung weight at each corner with the heavier 20" wheels/tires which will adversely affect the cars performance. Maybe more interior road noise? Rougher ride?

A "taller" wheel/tire combo (with increased circumference) will effectively give you a "taller" final drive ratio (less revolutions per mile) which could slow down acceleration, but (may) add some to top end speed.

 

I mostly agree with what you said but want to add something about the weight. The 2 lbs of extra weight will be compensated for by the decrease of 1 or 2 lbs for the tires. Lower profile tires are a little lighter than larger profile tires.
In all my years of plus sizing, I find no appreciable problem regarding weight or taller tire. The one issue that can't be ignored is the harder ride and increase road noise. Choosing the right tire can compensate for these as well but not make them go away.

I say go for it.

 

Not trying to be too big a nit here, but the size/weight of the wheel does not have significant impact on the "power" delivered. What it does is impact the acceleration. Basically, the motor puts out some power. There are power losses at a few points on the drivetrain, but the wheel is bolted on...little or no loss there. The larger diameter wheels do hit you from an acceleration perspective and it can be a bit complicated as you are both trying to accelerate the wheel from a rotational perspective (larger wheels hit you hard on this front due to more mass further from the center of rotation leading to higher moment of inertia) and get the wheels moving forward (total mass is all that maters here).

Sorry...enough of that...

 

traction loss is what you should be concerned about, they will look baller though.