Yes, Porshes have brembos. But the chance of the rotors from a Porsche fitting your G35 are slim to non-existant. Not to mention the price!
Yes, you can paint them, but then be prepared to either pay out the wazoo to get someone who knows what they're doing to take them off, paint them, and put them back on (uh oh, there's that pesky dollar sign again!), or take them off yourself, get all of the rust off, paint them, and put them back on. Then repeat this after they rust again 6 months later (this comes from experience!).
If you REALLY don't like the rust on your ROTORS then you can get CADIUM PLATED rotors to replace the perfectly good rotors Infinty put on there for you. They're not too expensive and it saves you the hassle of repainting your rotors every 6 months or so. You'll pay somewhere between $70 - $90 per rotor. And, yes you can get cadium plated rotors that are not slotted or drilled, which I would do unless you plan on auto-X'ing. Good luck!