You should be getting about 70-80MB/sec with USB3.0 on these drives. I assume it's a 500GB-750GB drive? USB 3.0 is 5Gbps (That's 640MB/sec max) so your drive will be the bottleneck.
I would get
this (If you also need FW800, this also comes in silver, which the latter does not) or
this. The black one is a bit ugly in the second link, but it is cheaper. I'm sure it looks nicer in person. Also the logo can be easily taken off. I just rubbed it off of mine (I have the FW800/USB3.0 version) and it came off easily.
You can always pop in an SSD in this and it will fly on USB 3.0.
Also if you are getting 28MB/sec on your current enclosure, it's definitely running at USB 2.0.
Your MacBook Air definitely supports USB 3.0, so it must be the enclosure.