For my own use, I'd happily run 17" permanently as on the road I don't push it so the difference in performance (regardless of speed rating) is negligible. Even when I start taking it to the track I'll hardly push it to the car's limit as I'll hold that last 10% back to preserve my daily driver.
I won't argue that larger wheels are better for performance, that's a fact. But for my own personal uses, I have no NEED for 18" alloys (I got mine at a bargain price so couldn't say no!) but if I were to buy new wheels now I'd probably opt for 17" to save a fair amount of cash.
Same reason I'm going for the NQSBBK over the TTRS calipers. Yes the TTRS brakes are better performing but I just don't need that level of performance.