You are are really talking about two different things, your internet connection speed and your wireless speed.
The numbers you are getting from speedtest.net are you internet connection speed. This is measuring the throughput you are getting between your computer and your ISP (Cox). These should be roughly the same if you are wireless or wired. Incidentally, these are pretty good numbers.
For your 802.11ac connection (between your computer and your router) you can see this somewhat. In the upper right hand corner of your iMac screen you'll see a bunch of icons in the menu bar. The one that looks like a series of diminishing arcs is your wireless connection. While holding the Option key, click on the icon. This will give your some info about your wireless connection. The Transmit Rate is the most interesting as this is the carrier rate of your connection. For me on 802.11n it's 300 Mb/s. I presume your number will be higher.
Now, that doesn't really tell you what your throughput is like. Unfortunately I don't know of any utility that can do that. But if you have two computers on your home network you can do some testing yourself. For instance, copy a large file from one computer to another and time it. Then divide the size of the file by the time. There are a lot of variables here (for example, if both computers are wireless, their distance to the wireless router, if they are doing anything else while the copy is going, etc.) but this should give you a rough idea of your throughput.
I hope this helps.
