[Ricesmoker]

Technically... Not EXTREMELY. You have a theoretically higher top speed with larger rims...

But the main thing is ride comfort on stock suspension goes down a BUNCH.

I don't know what to say on the traction... Because yes, a wider wheel will give you better traction.

Well to correct some slight misinformation:
20" rims dont have a "theoretically higher top speed". That is a funny statement. You dont drive on your 20" rims. You drive on the tires that are on the outside of your rims. So the diameter of the rim has no advantage to a higher speed. Top speed is determined by the overall diameter of your tire. 27" is stock. If you put on a taller tire you would increse the terminal velocity of the tire. If you get a shorter it will decrease that velocity.
The other factor limiting top speed and acceleration is that turning a larger rim, that is turning a heavier rim, takes more energy.
So bigger rims weigh more and will reduce you speed or acceleration. Most 20" tire/rim combinations are on the heavy side. So if you have an average combined weight of 60 pounds, that does not compare well with the stock 17" bullit rim with an ET Street weighing in at 38 pounds.
Traction will always be better with a tire with a taller sidewall. So you guys that run 30, 35 series tires on 20" rims have very little sidewall flex. The ability for the sidewalls to flex is critical to launching a tire from a dead stop. That is why slicks have huge wrinkle walls. So you can buy a wider 20" tire but the lack of flexiblke sidewall will hurt the traction compared to a similar width tire of the same height with a larger sidewall.
I run 18" Boyd Coddington rims on the street. I race with 17" OEM bullit rims.
I save 94 pounds of rotational weight by switching rims for the track. (Pirellis front/Mickeys rear)
So to answer the original posters question - yes 20" rims will hurt performance and acceleration to some degree.