Personally, I would not have a unit modded unless I'm told what chips are replaced. It is crazy to let someone get "under the hood", and make some potentially useless changes, and then not tell you what they did!
I did my own mods, and I believe they are better, but I don't have any way of knowing. Well, I have anecdotal evidence. In particualr, for the R4, I remember what it sounded like before and after. I also have a modded unit (mine) and an unmodded unit (a friend's) with the same mics (fixed) in the same room. Different show, but I'm pretty convinced, for the R4 at least, that the mod makes it more detailed. This was with Countryman B3 lav mics hanging in front of the stage.
I guess if I wanted to be systematic, I would mod one channel. Then feed both channels with a small omni (like Countryman B3), and do some *ambient* recordings with mics side-by-side. Unfortunately, I was too impatient when I did my mods. I just ripped the unit apart and modded both channels! For me, the cost was minimal, just $20 or so in parts.
Finally, I expect our mods to have more effect on portable gear, where the rail voltages are lower. For example Edirol UA5 and R4 have +/-5V, and the Fostex FR-2LE has something like 8V (single sided). In these cases getting a good "low voltage" opamp in there should really help. At least that is my belief.
Richard