The T5p is an absolutely outstanding headphone, IMO. With that said, two quick notes:
First, while bass extension and quality is superb on the T5p, its quantity simply isn't (IMO). You can naturally EQ this up to taste, which will help out dramatically-- but it's something to think about. (And I LOVE the T5p).
Second, the recommendation for the Intruder is apparently a solid one (I haven't heard it, but others whose opinions I trust have said good things). But to take advantage of it, you'll need to have your headphones modified for balanced operation. You can either modify the termination of the cable (easiest and cheapest) or the actual connection to the cups themselves (better in some circumstances, but a bit pricier). It seems the T5p responds very well to running balanced, so this might be a great idea, but just understand it involves giving your headphones to somebody for a couple of days and shelling out some dosh. (If you end up going the cup-mod route, I've confirmed that both Moon Audio and BTG Audio can do the modification for a reasonable fee).
As for the isolation-- these should be fine for blocking out the sounds of people in the hallway, unless they're being REALLY loud. The T5p are certainly not the final word in isolation, but they do provide fantastic sound in a closed can, which makes them vastly more usable for situations like the one you describe.
As for other cans, if you REALLY want enough bass to give you an aneurism in a closed can, the Signature DJ would probably be a good choice. I did not care for them, but plenty of other people love them, and it looks like they have at least a claim to the title of 'Best Basshead Can Available'. I would say that although I think the SigDJ looks horrific, they are considerably more portable than the T5p (they isolate better, have removable cables including one with a mic for cellphone use, and the steroid-infused bass response works well when there's a lot of outside noise).
Honestly man, it sounds like you've got a heckuva upgrade on the way (no disrespect to the current gear, natch), and you should have some fun times ahead. If you've already laid down the cash, just let yourself enjoy the new toys for a bit before you start second-guessing yourself. If they don't do it for you, look for something else. If you love 'em, then just love 'em!!!
Let us know how you get on ;). Congrats on the new gear!!