If you have any concerns about the power or the feel of the Sprinter, I suggest you test-drive one. Then you can make your own decisions.
The big difference is your Sienna was a gasoline engine, and the Sprinter has a diesel engine. Worlds different. The Sienna has 266 hp at a high RPM--like where you never actually operate the vehicle; that's the nature of gasoline engines. Also, take a look at the torque--which is where it's at with diesels. The Sienna probably only has 245 lb-ft, while the Sprinter has 325 lb-ft. Once again, the Sienna has its torque peak at an RPM well above where you normally operate the engine. Diesels, however, make their torque and hp at RPM ranges where they'll run all day long; if you run a gasoline engine at peak hp or peak torque, you'll burn up the engine in short order.
The Sprinter isn't a dragster, but it has pretty decent performance all in all. I have a Class B motorhome which tips the scales at about 9600 lb when I'm traveling down the road, and I have no issues with the power at all with that. Although I'd like more power when I'm flat-towing my 5000 lb Jeep on interstate highways, I don't have real issues and I'm able to do it. I just don't engage in drag races.
Rock Doc
2014 Pleasureway Plateau TS in 2013 Sprinter 3500