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.
Jack,
This is exactly what I'm seeing as well. 300Mhz difference is clock speed translates into negligible increase in FPS. Like Phil mentions, if you benchmark real world performance for applications and gaming the differences are unperceivable. My device performs as promised/expected.