^reflashing the ECU will "help" see better performance, but will not necessarily add actual power, etc. Sort of "wakes up" the ECU. Technically messing with anything will only void the warranty IF a future problem is directly caused by the modification you made.
However, you have a g25. Even with a few bolt-ons, etc. plus a flash. Idk how much gain you're really going to see. I've gone through this with two infinities already - the first I ended up spending so much money nickel-and-diming every bolt-on that I could have just gotten a turbo slapped on. The new car, there really is nothing for it and the only way to see more power is serious mods like a turbo, etc.
Most likely you'll end up doing the same bolt-ons everybody does then gets upset that they only gained like 15hp max. (not enough to feel imo) On my old fx35 I ended up making like 235awhp on the dyne, compared to a stock 215(ish)… after spending THOUSANDS on bolt-ons, etc. I mean I could have gotten an entire, tuned turbo set up for the same money.
If your plan is to really make some power, save up and do the mods that are worth while. Little bolt ons here and there will not actually do much - it may change the sound a bit or make a whirring noise at the intake, but won't add much power.