Use the tag inside the drivers door. Remember that pressure is when the tires are COLD. Once warmed up and running they will increase 2-4 psi depending on your weather.Duh forgot to answer your primary question.. over-inflating your tyres could result in damage to those tyres and a rougher ride.. especially when going over bumps or tight turns.
Same here. Keep an eye on tire wear patterns and you can adjust accordingly. So far at ~34psi mine seem to be wearing evenly across the tread, but if I notice excessive center treadwear I'll air down a bit. I'd add air if I notice too much edge treadwear (unless you do a lot of hard cornering / switchback mountain driving- that'll eat the edges too).A lot of Aura owner's, at least online, have been running 34-36psi pretty regularly and recommend it. Yes, it does make it stiffer, but the tire max pressure is well above that and it helps with stability and economy, and is not overinflated.
I had 31 in my 18" GY's on my G6 for a long time and it was fine but I wanted to make it more stable and get rid of the flat tire look when parked and bumped them to 34psi cold. Definitely feel it, but I like the added starch in the ride, mostly.
Anywhere from 30-35 is A-ok, fine tuneable to whatever you like ride & handling wise.