I feel it's the day to day LEO contact that could be the bad part, not the defensive shooting with it. In a defensive situation, like others have said, that mag is going to be the least of your worries. Now a regular LEO contact could become an issue quick, it just takes the wrong cop and you have yourself in a legal situation. I feel like that legal situation could lead to your IA revoking your permit (temporarily or permanently) and maybe even confiscating your firearm (temporarily). Yes the extra 5 rounds would be really nice peace of mind, but not losing your permit is also equally important. I'd hate to be the one to test the situation, since it's uncharted territory it's hard to say what would happen.
Technically it's legal as its been pointed out, there's nothing saying you can't, but your definitely risking a possible situation. Whether you want to risk that as a trade off for possibly saving your life is your choice. I'm not the kind of guy to blaze trails so I wouldn't. I like to stay under the radar, my firearm and my permit is far too important to me to risk it. To me being able to carry only 10 rounds is way better than going back to being able to carry 0.