One idea might be to time installing a couple games before and after provided you have the space. Though this could get pretty time consuming and dash install performance might not be indicative of game access speeds. But at least it should be a consistent measure. Just delete in between and do nothing else to minimize any additional fragmentation. Level loading times may be another steady measure. Choose a couple games and check level load times.
I've had Reach installed on my hd for the longest time because I thought it was pretty awful without that. But decided to delete it in order to test the new disk format as installed it wouldn't even be accessed. And it didn't seem as bad as I remembered. Loaded some old disks like Forza 3, GTA4, etc. and was left with the general impression of somewhat improved DVD performance. I have other consoles, but they are different gen, and wouldn't be apples-to-apples for comparative purposes. Not sure if there is some LOD pops, or level pauses in certain games that you can reproduce that you might compare.