HKS AFR type?

[cre]

Oh, it's definitely a beneficial system and if it can be recovered I feel it should. But, it's only operated at part throttle when the car is up to normal operating temp. It doesn't affect your WOT performance except that the cylinders aren't cooled as quickly when you let up. The additional cooling may reduce ping as well allowing the ECU to push the timing a bit further. As the recirculated exhaust gas is inert there's no reason for your AFR to change whether there's more of it, less of it or if it's not present. Adding fuel and/or pulling timing serves to help cool things back down.

I think all of this is in the FAQ somewhere but I think it's probably time to revise it.... I think I can probably explain it better these days than I could have in the past.