I shot 115-gr 9mm, in SSP-Novice. It did not seem to matter at the IDPA match what I had in the mags, but next time I will load up with 124-grain.
(SOMEONE PLEASE CORRECT ME IF I AM WRONG ON THIS):
When I did my (so far, first and only) IDPA, as a first-timer they put me in Novice (expected), and shooting what amounts to a rock-stock-Glock, I was put in Service Pistol (SSP). No special optics, no compensators, nothing. Another guy in the group test-fired his 9mm (also 115-grain bullets) on the steel poppers, just to make sure they fell; they did.
Now, when you begin getting into the higher categories, your ammo will get scrutinized. In USPSA (IPSC), there is something called Power Factor, which is based on a mathematical formula of bullet weight times muzzle velocity, and is broken down into Major PF (usually the .45ACP and .38Super race guns all tricked out) and Minor (which would include 9mm). But as a beginner, you will pick up all of this stuff as you advance among the categories.
Whatever gun you bring, and whatever ammo you have with you, will dictate what category you will shoot in.
As a reloader, I have the liberty to tailor my loads how I want them...this is a luxury not every shooter has. So guess what? Bring what you can buy at the store