Diesel Sprinter downshifting to slow question ?

I assume you are referring to the Sprinter diesel? Diesels without an exhaust brake don't have a whole lot of "drag" when downshifted. The brakes cannot be installed at least on the 2008 on up V6 Sprinter, or the newer 4 cyl Sprinter I'd bet because of all the emissions equipment.
We haven't had any issues with either of the Sprinters we've owned in the mountains. Just run in 3rd gear, keep the rpm's below 4k, (the V6), and brake hard for short periods to keep the heat down