Would the time saving be worth it anyway?
I fly a 146 and we have to be at 250KIAS by FL80 so asking to keep the speed up for a extra couple of thousand feet in the descent is not worth the extra R/T time in requesting it!
You will probably still have to do the Kent scenic tour at 190KIAS so why burn the extra fuel in keeping the speed high for just a couple of extra minutes? Surely it can't be worth the R/T call?