802.11ac is a new standard which is an improvement over Wireless N. I don't think it is an official finished standard yet but many companies are adopting whats out there now anyways.
You will notice there are not a lot of AC products out there currently and this because the standard is not finalized.
Your Uverse internet is not going to slow down because of Wireless N. I have wireless N at home and a 150MB internet connection and wireless N does that speed without issue. Uverse internet speeds are a lot lower than 150 mb/s, so it won't be an issue.
And yes Uverse works with Macs, even Yosemite, in fact I am using it now.