If you're terrified of changing the bit rates on the DVD don't be. The Big Kids change the compression and bit rates on theatrical movie DVDs scene by scene--more often if required.
That "120 Minute High Quality" thing is a series of best guess presets that Apple scooped into one setting. If you dig down far enough, you get to the raw tools where you can set custom multiple pass, max bit rates, average rates, I frame placement, etc. etc.
This also works with the compression markers that Final Cut lets you burn into the show. I'm on shaky ground here because we've never actually used this for anything, but I know the theory. If you know that the story is going to go from high quality mid afternoon and low action to violent action at night, you can chance the compression appropriately on the fly.
Yes, you do need to do that for best results. A two-hour movie will not fit on a DVD if you don't.
Koz