Double stack .45's are coming out. SiG is rumored to be jumping on this recent bandwagon. Round capacity is purely speculative and it actually wouldn't be double stack in the conventional sense. The concept was (with the P221) and is rumored to be a staggered column magazine arrangement but keeping the exterior dimensions similar to the P226 (which would alledge a 10 round standard capacity). Several of the P250 incarnations are listed on the SIG sight with a .45 option. I seriously doubt that a double stack variant of the .45 will emerge in this series.
If you want a SIG, then get one. If you want a high capacity .45, look into the: HK USP45, Taurus PT145 Pro, CZ-97B. However, even these do NOT come in your desired 13-15 capacity with standard mags. They are all 10's with the exception of the HK which has an aftermarket mag with a capacity of 12 rounds available. The caveat is, it's made by ProMag. 10 round mag capacity would make many happy in competition shooting as the .45 platforms wouldn't HAVE to go with extended mags (ugly and cumbersome) to keep up with some of the 9mm platforms (usually restricted to 10 rounds even if their mags will hold more). If you need or want more .45 than is currently on the market I see you have 2 choices. Build it yourself or get a Tommy.