Yes, changing the seat will present the volcano with a surface that's no longer indented, and therefore fractionally closer to the knife edge. That means the mainspring won't be compressed as much and will seal sooner at lower IP.
But for the record, if you have a spring from which you just can't get a low enough IP, you can indeed shorten it. Using a belt sander, and careful placement of the top or bottom flat against the belt with low belt speed and frequent water cooling to prevent changing the temper, you can take off 10/1000th's of an inch and lower your IP 5-8 psi, with about 3 minutes work. It's a strictly unauthorized repair, but done carefully, it's an easy, safe solution to the problem, especially if getting a new mainspring is no longer possible.