I wouldn't want to encourage speeding on the Autobahn, but by US standards, German fines are hardly "hefty." Unless you fail to see the "80" sign and run into a speed trap with 160kph, the fine will be negligible. Some Autobahns are pretty heavily regulated, with "60," "80," "100," signs etc. being assigned almost randomly to fairly short stretches. Constantly watching for these signs can be quite bothersome. So don't worry about going too fast all the time. Anything the average US traveller is used to (going 5-15 miles over) won't cost you more than the average parking ticket in the US, and much less than a speeding ticket in the US.

Fines are heavier for exceeding the speed limits in cities, and for doing really dangerous and stupid things such as passing in a curve, going too fast without keeping a reasonable distance, and, common practice in some US states, not granting pedestrians' right-of-way at crosswalks.

As for your trip plan, I second the suggestion of A-95 to Garmisch. Do this for a day trip (avoid weekends) and either (a) continue highway B-2 to Mittenwald for some great alpine scenery, or (b) go to Neuschwanstein castle, about a 90-minute trip from the end of A-95. On the return, consider stopping at Kloster Andechs, an old monastery with a fantastic beer garden (about a 15-minute drive from the Starnberg exit of A-95). Don't drink too much, though, drinking and driving is one of the stupid things for which fines are indeed "hefty."

Finally, if you'd like to do more Autobahn driving, A-92 mentioned above is fine, but it may take you a long time to get to the open stretches, depending on traffic to the airport. Try A-96 instead. Once you leave the greater Munich area, there are very few speed limits. Also, this freeway is more interesting than A-92, with more curves, hills, etc. It turns into a highway after about 50 miles, so that's a good indication to turn around.

Enjoy your trip!