The thing I question is how much cargo you are going to put into a fifth wheel! 800lbs is not much for a family. I say closer to 1500 minimum! I`m at 2000lbs more than the dry weight loaded for a trip.
So loaded you will be 11000lbs. that will put your pin at around 2500-2700lbs. not much left for hitch and passengers and other things you will want to put in the bed.
As far as towing it goes....your current Sprinter is very low and aerodynamic. the fifth wheel is a lot taller and will catch a lot more wind! you wont see the 8-10 your seeing now. and you may not be happy with the performance, but there is only one way to know for sure!