If you want to swap out thermostats, a 2008 G37 has one that's 10 degrees colder. I've got a csf radiator/condensor for sale and I'm local to you. And I don't apologize for shamelessly whoring out my FS ad.
In all seriousness, it's the only real option. CSF is good. GTM has a better one in theory, but good luck getting them to send you a product. Mishimoto is Chinese garbage made by bestiality enthusiasts.
Make sure your oil cooler is 34 rows. I assume you're putting in a boost gauge and wideband and you just didn't mention it. What size injectors and fuel pump? Many people run new wiring to the fuel pump to prevent voltage drop. It's not a bad idea.
Who's tuning it? The canned tune Stillen sends you is horrible.