I recently bought my first subcompact autoloader, a Walther P99c in 9x19, for times when my full-size PX4 is too big to conveniently conceal. The standard mags hold ten rounds and are available, if somewhat pricey. I'm thinking about getting a couple of 15-round mags for the full-size version and using the sleeves available from Walther, similar to the setups available for the PX4 subcompact and the M&P compact, and using them for my reload mag. The obvious advantage is that it gives me an additional five rounds on board vs. the standard mag. Cost-wise it's a wash because I can get a full-size SW99 mag and the sleeve for about the same price as a P99c mag.
Is there a downside to this, or am I looking at a free lunch?