Check out Jotech Motorsports in Garland. They've been around since the early 90s and deal with a number of Japanese cars (and some exotics, one of their shop cars is a TT Gallardo). Since the 350Z/G35 platform came out, they have become very Nissan-focused, and should be comfortable with a G37. These days they have kind of become 'GT-R tuning central', but if you give them a call I'm sure they'd be happy to have your business.
See also Cobb Tuning in Plano. Cobb has been around for a while, though their Plano facility is relatively new. Their specialty is factory-turbo cars (ie Mazdaspeeds, Subarus, Evos, GT-Rs, turbo BMWs). I haven't had anything done there yet but friends with 370Zs have been happy with their work.
You could also check out 5th Gear Motorsports, they're reputable and can work on pretty much anything. They're in Lewisville IIRC.
Or if you'd prefer a dealership, I've gotten almost all of my 350Z's significant maintenance stuff done at Courtesy Nissan in Richardson. I've never had work done at an Infiniti dealer before but I would assume the hourly rate is cheaper at a Nissan dealer...
[This message has been edited by madd_ag_05 (edited 4/26/2012 4:51p).]