If I purchase 19" rims, will I get better or subpar performance than over 18" rims? I've talked with some other people who seem convinced that 19" rims are all show and no go. I want to keep functionality and form, so should I stick with 18s or get 19s?

 

I don't think you will notice any performance loss going from 18 to 19". - just make sure the wheels are light.
If you plan on using the wheels for autocross or something, you probably want the 18's for more tire sidewall.

I have 19" TE-37's with fat 275 Toyos in the rear, and I chirp the tires a lot more now between 1-2 shifts than when I had the stock 17's - if that means anything.

 

if you are going from a track model, you will definitely notice the difference. I think going from the heavier 18s wont be much difference though.

 

I had 18"s and now have 19"s. Not easy to compare as apples to apples unless the tires are the same. If you go with the same tire then it's close. Then there is tire size. My 19"s have a lot wider tread than the 18"s I had and the tires compared to stock......I won't even go there.

Bottom line is "With the right setup" you will be happy with either. The 18"s with the right tire and light rims will probably perform a little better on the day your at the track. The 19's IMO will look better everyday.

If you are concerned about performance at all.....GET RID OF THE STOCK TIRES!!!!! I would go with the size that looks best and spend some coin to get the best tires.

 

Yeah, I was considering getting rid of the stock tires, but right now money is tight and I am more interested in getting go fast features at this time.