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Does anyone know how much stock 15" Alloy rims weigh.. the ones that come with the Si's in canada.

 
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20 lbs according to the boards at clubsi.com
the 99-00 ones (also the 03 EX's) are 16.5

 
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I was thinking of getting some 16" Motegi MR7s.. originally wanted to go with konig tantrums but they are only 17" + and the mr7s are basically the same design but come in 16s. Will there be a noticable difference in performance between the stock 15s and the mr7 16s (20lbs)?

 
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20 lbs is 20 lbs. I doubt the inertia from the extra 1" will make a noticable difference in performance, but if you want performance, there are plenty of rims that weigh less than 20lbs. Those Motegis are must be filled with lead or something. They're rediculously heavy for such a simple rim. I'm not saying to go out and buy a $400/rim set of Volks either. You can get respectably priced rims that are light and good looking for fairly good prices. Wheelspecs.com ... look through there, they list tons of rims. The other thing that you gotta realize is that there isn't really a 16" tire that lines up well with the stock tire diameter. The closest size has 2 or 3 choices and they're all over $150/tire. So you might want to consider that.
Read the wheel forum too... there's tons of stuff to read about rims in there. But if you're looking to make your car faster (albeit a very little amount), lighter wheels are the way to go. the less weight the motor has to turn, the better, and it means less unsprung weight... which helps your handling.