My current bike comes with 25s and wouldn't even take 28s in the rear. The tires that came with the bike are 130 psi clinchers and you need more not less psi to handle more weight so you'd pump'm up the spec allowable on the sidewall. I looked at a pic of your bike on the web and it looked like it had a straight seatpost. If so, a setback seatpost probably will soak up more road jiggers and bumps than messing with tire pressure. And, from the write-up the Giant has an ingenious design that allows for 20 mm of setback instead of 5 depending on how it's set up.
Last edited by McBTC; 08-30-15 at 07:43 PM.