Soooo, after a brief bit of A/B from an RSA Intruder listening to the modded T5p in balanced mode, and the regular T5p via a single-ended connection, I have a few preliminary thoughts.
Firstly, the Jaben mod isn't just to the cable and cable termination at the headphone end - they've also changed the pads. The Jaben pads are much firmer and heavier, and between them and a much heavier cable I find them slightly less comfortable than the regular T5p with its super-cushy pads (took me a little while to work out why I was finding the comfort factor different). They may soften with a bit of use though, so I'm rushing to make final conclusions there. Because I can easily tell which is on my head by the fit I can't really see how I can do a blind and volume-matched comparison between the two, which is a shame. Will have to see whether they isolate any better as a result, I suspect they will actually. Will try and have an external listener to let me know whether there is any material difference in leakage. Then will try in an environment with more outside noise too.
As for the sound - the bass quantity and impact is very much increased, I think at the expense of a little tightness. The difference is honestly a bit startling. Stuff like Teardrop by Massive Attack, with the really low sub-bass that kicks in at about 40s is much improved, as were tracks like Super Massive Black Whole by Muse, where the kick drum in the intro should have more body and impact than it does out of the regular T5p. I need more time to listen, but I there is a subtle difference to the mids too, perhaps slightly less forward sounding now that the bottom end is filled in a bit more. I really like the forward mids of the regular T5p, as they suit my listening preferences, really helping guitars to "sound right" in rock and metal (to my ear as a guitarist at any rate), but I think overall the slight concession is worth it. Imaging also benefits a bit I think, listening to some Pantera and Tool, where both drummers do a lot of fine detail stuff with splash cymbals and bells, that's slightly easier to pick out on the modded ones.
Rather curious as to how much the extra power per-channel in balanced mode is contributing to the difference in bass, since the bass is one aspect that noticeably scales with better amplification, and how much is the pads alone. Will have a listen to the modded T5p single-ended at some point to try and separate out the contribution each is making to the whole. Offers the intriguing possibility of a simple modification to the pads for people who'd like a little more bass.