BMF and I have been working on some ideas about pads. He has concluded through testing that the 840 pads make a MASSIVE difference. Isolation over stock is due mostly due to the seal. I would reocmmend getting 840 pads ASAP. They are on sale at Amazon currently, with free 1 day shipping by using Discover. I personally hate the 840 pads, but am willing to reconsider them due to how easy BMF has made tuing with them.
Follow BMF's DBV#3 to get an idea how to squeeze everything you can out of a T50rp. Using his guides will help familarize yourself with the ins and outs of between different tweaks. If you haven't read this thread, I would encourage you to: http://www.head-fi.org/t/618659/fostex-t50rp-incremental-mods-and-measurements#post_8535316
I just got the Shure pads today, and man, were you right! I ended up putting a little cotton in the cups, along with sealing them, and they sound great. Now we'll see if I can leave well enough alone...
I don't think it can be overstated how much of a difference the pads make. It really completely transforms the cans. Definitely a must have upgrade, imo.