Using my script and a DRC meter (see here) I conducted a comparison between my original mp3s and new high-quality mp4s received from iTunes Match. The original mp3s were sourced mainly from my own rips (using Lame from 2000-2004 and iTunes from 2005-onwards usually encoded with VBR 192) with a small number from napster, audiogalaxy etc.
Each old song file was compared to the new song file by measuring the dynamic range compression (DRC) of each in turn. The differences between the new and old values were calculated and the results appropriately binned before plotting the frequencies at the bin midpoint. Briefly, for each file a low or negative value indicates less compression used. So negative values plotted indicate iTunes Match versions of songs that are less compressed than the originals.
For a large sample (~3k songs) of my iTunes library the graph shows the iTunes Match versions appear to demonstrate higher DRC (more positive values). The other thing of note is the large number of files where no apparent difference is shown.
This analysis is not perfect – most of the measurements are subjective but the differences are there even if slight.