I just made a similar trip about a week ago, I left the Welt around 2:30pm and arrived late in the evening because I wasn't rushing to get there, as well as hit some rain storms that required stopping along the way due to visibility. My original plan was to stop in Innsbruck and then to Stelvio, but ended up changing it and going directly from Munich to Stelvio (typically about a 4 hour drive). Since you're stopping at Innsbruck, if you continued on to Stelvio, it'll be dark by the time you get there (depending on how long you're staying at Innsbruck). As suggested earlier, you want to see Stelvio and those roads during the daytime, it's really worthwhile.
There are not a lot of unlimited speed sections from Munich to Stelvio, and in fact you'll be going through a lot of towns along the way which requires you to slow down. You won't be exercised too much to "not follow" the break in procedures. There was a few areas with construction too that created some traffic congestion, but not to a point of stop and go traffic, just slower speeds. I hope that helps you make your decision. If not, here's a teaser pic of what you can see in the alps