My surface pro 2 with 4200u has no problems turbo boosting to 2.6ghz (at least this is what task manager shows) and it shows in device manager as 1.6ghz to 2.3ghz.
Well that is most interesting then. I've been doing some stress testing to see what happens under full load, and pure CPU stressing has it throttle once 80 degrees celcius is hit such that it levels off to 2.4-2.5 GHz. CPU + GPU stressing has the CPU downclock to 2.0 GHz. Both tests were performed with HeavyLoad (http://www.jam-software.com/heavyload/) while temperature was monitored with Core Temp (http://www.alcpu.com/CoreTemp/).
These results and the above comment on the 4200U would indicate that, under real world situations of 100% sustained CPU usage, there probably wouldn't be any discernible difference then due to the throttling of the 4300U. Even if you could hack to unlock the clock speed/throttling, you'd either run your system dangerously hot or you'd still want to throttle it.
I'd be interested to see what similar tests with the 4200U would produce.