Actually this is technically not true. The M11-A1 is based on the .40/.357 P229 frame that uses a wider mag. The A1 gets its 15 rd capacity by using a wider P229-1 mag instead of the lower profile follower and spring in the previous 15rd Mec-Gar mag for the P228/P229, even though I believe Mec-Gar also makes the M11-A1 mag. Also, there is the problem of locating a stainless steel slide with full height serrations that will fit the P228 and older 9mm P229 frame.
However, I am not saying the M11-A1 is better, just slightly different and not easily replicable using an older P228/P229 frame, assuming someone really wanted to do it. The M11-A1 is probably as close as you can get these days if you want a new P228, short of finding someone's safe queen on GunBroker.
Yes you are 100% spot on the M11-A1 is basically a P229 without the rail and full height slide serrations. You are also correct in terms of the mag but the 2 Mecgar designs are functionally identical. People have been using 15 round Mecgar mags in the P228 & P229 with the lower profile follower with outstanding results for as long as I can remember.
As far as the width of the frame & stainless steel slide why would anyone want one on a P228. The beauty of the P228 is its width feel in the hand and its balance. Many people like myself who cut their teeth on a P228 find the P229 top heavy and thicker in the hand. For some this might be an advantage but for those who truly want a P228 the stainless steel slide and P229 frame make no sense. No you will not have full height serrations but I do not consider that a deal breaker.
Again if you like the configuration of this frankenpistol are are willing to dish out the $900+ I say go for it but please remember you are not buying a P228 or a M11 or even a M1-A1 because they do not exist. What you are buying is a P229 with no rail full height serrations, night sights, SRT trigger and a lot of made up marketing hype.
My point is that you can take a P228 keep all the characteristic that make it a great pistol add a SRT trigger for $50, Mecgar mags at $25 a pop and come out way ahead of the $900 price tag Sig is charging. The same can be said about the older non railed P229 except you loose the full height slide serrations.
Anyway you slice it the M11-A1 designation and package is a marketing ploy which deceives those who do no know any better into believing that they are buying a elite mil-spec pistol which they are not. I personally see no appeal in it. YMMV