My car: 2010 G37x Coupe - with Stock S wheels (19")
Spacers are for the front wheels only.

 

H&Rs get consistently good reviews, Ichiba is another decent option, both should come w/ extended lugs.

 

Not planning on changing the studs.

 

I've got 20mm H&Rs all around right now, I've had them on for a few months and I haven't had any issues with vibrations or the nuts coming loose. I have the DRM version, so the spacers bolt on to the existing nuts, and then the wheels bolt on to new studs that are mounted on the spacer itself. It's super solid, and they've tolerate some pretty high speeds, and hard braking and cornering. Eibach spacers would be another high-quality option that should put up with track use and other abusive conditions. I've read mixed reviews about Ichibas myself, and I do remember someone having a wheel fly off from this forum while he was using Ichiba spacers. It's also very possible he didn't install them correctly or torque them down to spec. Also the Ichibas can sometimes have issues with vibration, and people have solved it with some copper wiring acting as a crush washer at the hubcentric ring between the spacer and the hub.

As for your intent to not replace studs, you may not have a choice with spacers as thin as 5mm, since the plate would be too thin to have it's own studs. And shoot, it might be too thin to even have it's own hubcentric ring to help support the wheel.

 

I second H&Rs. Its the only thing we currently offer. They make top tier products

 

Can never go wrong with H&R's quality. Top notch.