Regardless of how much content the Xbox 360 discs hold or whether the HDD copy is just a disc image, having the image on the hard drive should enable the Xbox to read the content from the disc image on HDD rather than from the optical disc.
In fact, I know it reads it from the HDD when I'm playing because the optical drive spins up briefly when you insert the disc, then stops and doesn't spin up again until you swap discs, presumably to verify that you have the other disc. If it didn't read the game data from the hard drive, what would be the point of installing it to the hard drive in the first place?
Any in case you think that the HDD copy is just "in case your game breaks", what good will that do you when you go to launch the game from your HDD and are presented with a "Please insert disc X" dialog as soon as you load up a game?
Lastly, for the people without hard drives, this is a moot discussion because they have no choice but to swap discs.
For those of us *with* hard drives who have installed *both* discs onto the hard drive, it's immensely frustrating to have to swap discs over and over again when it doesn't actually need to read the data off the disc. I understand the need to verify you have *a* game disc when you start the game, but making you pop in whichever disc the current mission is on just so the Xbox can verify you have that particular disc, then go on to read the data from the hard drive is just asinine.
If they really, really wanted to make sure you have both discs, they should make you insert both in sequence when you load up your game and then toggle a flag in memory indicating "yes, he has both discs" and let you continue your play session without ever swapping another disc.
Please, please, for the love of the gods, patch ME3 so I don't have to get up every half an hour to switch discs when all the data is already spinning on little magnetic platters inside my Xbox! If this sort of thing continues, I will have to avoid ever buying multi-disc games for the Xbox ever again. PC gaming was never this much of a PITA, and when it is, it's easy enough to work around the DRM bullshit.