Yeah, 17's still look really good. It's your best bet really. The bigger and heavier the rim is (unsprung weight), the more power it takes to spin the wheel. Even if you just save 2lbs from your stock wheel, you are really saving something like 40 lbs of unsprung weight, which can help acceleration alot. Someone school me here if I'm wrong.
Really though, the best option is getting a 16" forged wheel, but... everyone knows 16's aren't exactly flattering, especially if you are going aftermarket. Like I said, it's up to you
Something to note though, if you can get those Mugen GP's off of CollegeHillsHonda for $1800 still, I'd definitely do that. They are super cheap for a forged wheel, super lightweight, and look amazing on GG. You save something like 6lbs per wheel off the stock Si wheels, AND they are 18"s. Best of both worlds.