Since you seem to be set on slipstreams, this shouldn't be a wheel brand discussion, but there are better options for a little more money IMO.
I'll just lay down some things for 16" wheels and tires VS. 15" wheels and tires. Naturally, and in most cases, a 16" wheel will fit bigger brakes than a 15" wheel. But do you even need bigger brakes than whats on the stock ek9?(which are already great for the car's size) If you're doing endurance high speed racing, sure, but for street driving and normal track events, the stock ek9 brakes with good pads can max out street tires just fine. When the tires you are using (all street tires) can be locked up easily with the good ek9 brakes, a larger brake setup gains nothing but heat soak ability. And if you want the light weight aluminum hub factor of aftermarket brakes, theres an 11" 4 piston wilwood setup available with a seperated (but of course connected with bolt) hub and rotor.
I think a 16" wheel is the best size to compliment the EK's size as far as looks go, because at some angles a 15" looks small and others it looks great. But thats up to you.
If you keep the same overall wheel and tire diameter there should be no changes in top speed or final drive ratio. If you want to deliberately use a larger or smaller tire to change the final drive ratio you could do so, but keep the OD the same unless you really know and want the outcome.
Unsprung weight, especially that of the wheel and tire, is the best weight to save on the whole car. So you want the setup to be as light as possible without making sacrifices in strength or grip. 15" tires of the same brand are lighter than 16" tires. Usually about 2-3 pounds or so lighter. A 15" tire side wall will be slightly taller (thus, less stiff) than the 16" tire side wall, which is good for straight line accelleration but slighty worse for cornering. However some tires are stiff enough in 15" size anyway, as some tires are built to be stiff. A taller side wall will be softer and absorb road irregularities better than the stiffer side wall, if comfort is an issue. 16" tires are more expensive than 15" tires, but there is more tire selection in 16" as far as street tires go. As for race tires there are plenty in either size. For tire sizing, a 205/50-15 and 205/45-16 are typical for a 7" wide wheel. 215mm wide tires are harder to find in my searching, and if you run a 225 for extreme grip you should use at the very least a 7" wide wheel, preferably 8" wide.
16" wheels are heavier than 15" wheels. This of course equals better grip, accelleration, braking, even gas mileage with a 15" wheel. A heavier wheel setup is harder on driveline components as well. Another thing people don't think about is how a 16" wheel+tire's weight is more towards the outer edge than with a 15" wheel, which increases the leaverage of the wheels weight, making the whole setup harder to stop and accellerate. This of course it not due to the tire being bigger, but the rim's (not wheel, rim is the outside part of wheel, wheel is the whole thing) mass and tire bead being more towards the outside.
16" wheels will have more open area for brake cooling than a 15" wheel of the same size. Wheels with plently of open area between the spokes are preferred in racing due to the fact that they will cool the brakes better.
In closing, I vote 15" wheel, if your concern is performance. Just make sure you pick a stiff and sticky tire. If you care more about looks, a 16" wheel is it.
) you will have more top speed but that's only the one side of the coin 
i have 15's but wouldent mind a second set of 16 just for fun 
the only reason i dont really respond is cause your right so i got nothing to argue against 
