nope When the Xbox 360 received the "New Xbox Experience" Dashboard update allowing gamers to install their titles to the hard drive to improve load times, it actually made Halo 3 perform worse than when running off of the disk.
Unfortunately, this is exactly how the digital copy of Halo 3 functions. Kotaku spoke with Bungie community director Brian Jarrard about the issue, who revealed that to Bungie's knowledge "no additional work done on the code to optimize the game for HDD [harddrive] installation." Brian is quick to put it all in perspective, though:
The game is obviously still playable and enjoyable and it's likely most people may not even notice a difference. However, we did encourage people who owned the disc to not install to HDD since it would result in longer load times. Keep in mind that we're pretty hardcore about this kind of thing so when we found out that it loading Halo 3 to the HDD would increase load times, we wanted to make sure our fans knew about it.
Bungie is currently hard at work on Halo: Reach, with the multiplayer beta set to begin on May 3.