Finally got around to installing the NGK-R7437-9 plugs and they feel more responsive than the NGK-R7437-8 plugs. I had 15K miles on them, perhaps that was the difference (new vs used). Either way, I'm happy with these plugs. The old plugs had a uniform wear pattern, which was nice to see.
When my car was first tuned to stage 2 just over a year ago I had the NGK 7437 plugs installed "racing plugs" recommended per APR website. They lasted 15k miles and were great but pricey. This time around I ordered the RS7 plugs, $52 from DAP with bonus points. Honestly, driving around with the RS7 plugs for a couple weeks now, the car does feel a tad less manic, a tad bit slower, a tad bit urgent. I really didn't think there would be a difference at all but there is in my experience between the 2 sets. Now, I did save $100 this time around and the car is not slow but it really does feel just a slight bit off from running the racing plugs. Nothing wrong with either set of plugs for tuned cars. If I happen to have an extra $100 to throw at plugs next time around I will likely get the 7437's again. Not sure if the gapping on the RS7 plugs is what is making the difference in feel and performance. I have them gapped at 24 just the same as the racing plugs were at the time of use. Just my 2 cents from personal experience.
For Stage 1 and 2 cars, you don't need those NGK race plugs. Don't know why APR is pushing that on you folks but they are wrong in doing so. Those race plugs, heat range 9, are meant for Stage 3 and 3+ cars.
Last year I ran the NGK 7437-8 plugs with my APR Stage 2 (v 2.4) and I never really liked them. Idle was lousy, and just always felt "not right" to me. After about 13k I changed to Brisk ER12S, and I like them much better. Upgraded to flash v 2.8 (more timing I think) and still feels great. Even if they only last 10k it's well worth it. And they're cheap.