As far as i can see, the rate limit is designed to limit the speed of data link layer. And the speedtest or other tool to test the speed is depend on network layer. So it is normal when the test speed is the 1/2 of your setting value. Also you need to consider the wireless interference so you may see 1/3.

While that sounds reasonable if fails to explain things due to the following:
1) The upload is exactly the limit set, if it was as you say then that would be off the same as the download. Removing your data link layer hypothesis.
2) When I remove the rate limit then the test reaches the maximum speed the network is capable of within a few % for overhead when compared to a wired connection. Thus eliminating your wireless interference hypothesis.

So still not convinced that things are working as they are supposed to. For now I simply set the rate limit to 3x my intended download and 1x my intended upload and everything works fine.