I do believe DC-HSDPA is capable of up to around 42Mbps so 28 can be expected a lot of the time I guess in excellent conditions. 3 do have a throttling system in place to balance out congestion so it will vary but in ideal conditions you could expect to get 20Mbps+ easily.
Of course it doesn't change the upload speed, just download. 4G upload is insane.
Wiki said:An advanced HSDPA+ network can theoretically support up to 28 Mbit/s and 42 Mbit/s with a single 5 MHz carrier. Doubling the bandwidth to 10MHz results in theoretically, double the sustained data rate
Of course it doesn't change the upload speed, just download. 4G upload is insane.