If you're still having this throttling problem (I have also, for about 2 years now) I finally found a solution. Open up ThrottleStop, click FlVR and check "Disable and Lock Turbo Power Limits". Finally an end to the constant 0.8 GHz throttling!

I think Dell must hijack those TurboBoost power limits as a way to massively underclock the processor and try for better advertised battery life.