I own a Beretta a 96 (40 s&w) that came with ten round magazines. I've seen 15rd mags available for it,
Sorry, no. The 15 rd mags are for the Beretta 92 in 9mm, not .40. Your FULL capacity mags hold 11.
but are hi cap-mags needed for self defence?
Quite possibly! You don't get to choose the number of badguys who may well be hopped up on drugs, time of day, distance, etc, etc, etc, etc, etc. None of which lend themselves to least rounds fired.
I practice enough to consider myself competent but have no experiece's other than paper.
Hell, EVERYONE is "competent" online! I would suggest finding an IPSC or IDPA match near you and shoot your carry gear and see how you do. Unless you win it the first time out and didn't learn anything new, then yes, you're probably very competent.
10 rounds are enough. You will rarely need more than 2 or 3. If you can't solve the problem with 10 rounds you need instruction and practice.
Says who? Some nut on the internet? As stated above, you have next to zero control over many of the critical factors.
It's one thing to accept fewer rounds because you choose to carry a smaller gun. It's another thing to accept fewer rounds because some bureaucrat control freak decided YOU didn't "need" more than 10. (perhaps even citing drails post....)
And they are not "high" capacity magazines, they are FULL capacity magazines. 10 rd mags are "restricted" capacity.