Something else to consider is your purpose for participating in IDPA. Cater what you run with to what you are trying to accomplish.
Are you looking to try to be a competitive shooter? If so, yes, the 8-rounders would be a disadvantage. Go with the 10 rounders or a larger gun that has a longer sight radius.
However, if you are just looking to improve your skills using your firearm in a defensive manner, then it doesn't really matter. "Run" with what you run with. If you are going to be carrying an 8-round mag in the gun with a couple spares, then the more reloading you do in IDPA, the more practice it will be. Also, if you will only be carrying the handgun with no backup mags or you, then go ahead and bring the 10 rounders. They'll make you more competitive and it really doesn't matter if you get good at reloading or not.