19s...
 

how much are 19s going to slow the car down? alot? a lil bit?

Re: 19s...
 

it's not the size, but the mass that makes the biggest difference... basically, if you want performance, the smallest wheel you can fit over your brakes is best, always. if you don't care and want style, you can spend a lot of money to minimize the performance loss, or you can go cheaper and really have a lot of comprimizes for larger wheels.

so as long as my new rims/tires are the same as stock... ill be looking at minimal loss in power?

Tyre compound can come into play.

More accurate to say that if the weight and dimensions are the same, plus the contact patch and the compound are the same, oh and the aero effect of the wheel...

Add to the fact that a few pounds variation in tyre pressure can make a mountain of different.

To talk extremes, I can notice the difference when two near identical sets of bicycle wheels differ in rim weight by 50 grams per wheel.

You may notice that there's a number of riders that switch to 26 instead of 27 inch wheels on the climbs during the tour de france - wakeech is right, the less mass you have to spin the better.

Wheels are a great place to start if you want to improve a car, but the 18's that come on the RX8 are pretty good to start with - so expect to pay a lot for wheels and tyres that are going to make an improvement that you will notice .... or go thinner and smaller - not me :D

If we upsize the wheel, and keep the overall tire diameter the same, doesnt that mean a shorter sidewall? Wouldnt that reduce the weight of the tire? Or does the tire weight matter at all?

rubber or metal, weight is weight.

Think in terms of the old chocolate wheel. Try to spin a thin wooden one and it's fairly easy. Imagine how much harder it would be to spin if the edge of the circumference was heavily weighted. (and harder to stop too!)

Thinking along the same line, if all of the added weight was evenly distributed at the centre, the weight needs to travel a lesser distance in the circular motion, therefore a bit easier to spin up than on the outer edge. (Consider spinning a fishing weight on a piece of line ... how hard is it to spin it quickly extended 1 foot, 3 foot, 6 foot, etc.

The worst combination for performance is heavy wheels and heavy tyres. (Yes, heavy brake discs, heavy drive shaft, gears, rotors, etc) But remember the balanced mass spinning in the engine also helps to keep it smoothe.

Exotic light weight materials increase cost, reducing the amount of material used reduces strength and durability. It's all a compromise in the end.

unless you are really trying to shave tenths of a second on quarter mile runs the differnece with 19's will be small

I'd stay with 18s and increase the widths to 9s or 9.5s; mount 255/40-18s. If done right, you can gain extra cornering potential for no gain in weight. As an example you could do 9 x 18 Volks SE37Ks all around (40-mm offset). These are ~18 lbs each (compared with the stock 22.5 lbs for the 8 x 18). A 255/40-18 tire will tip the scales in 28 lbs range for a total 44 lbs for wheel/tire. Not bad considering the stock Rx8 wheel/tire weight is probably closer to 47 lbs (225/45-18 on 9 x 18, 22.5 lbs for wheel, 24 lbs for tire). IMHO, the 3000-lb Rx8 needs 10-inch (255 mm) rubber contact patches to claw at the pavement for maximum road-holding and feedback.

I disagree. The Rx8 makes so little power (compared with an FD3S Rx7), that 5 to 10 lbs extra per wheel/tire will put extra drag on the Renesis motor, slowing your accleration to a noticeable extent.

Shaving weight from the outer circumference of the wheel makes a noticed difference.

On my road racing bike (yeh, pedal), I can pick the difference if my tyres are ~50 grams heavier.

Since the 19s have been on I've noticed only a slight difference in acceleraton and braking. That difference will be make up with and exhaust and intake alone. Even after the third day of having the wheels on, I can't tell the difference.