I pushed some Plast-i-clay into the innards of the T55C (about 3 lbs) and I would like to report that it did make a positive sonic difference:

It cleans up the lowest and highest notes, while leaving the large and lush soundstage mostly intact. It's definitely worth doing. Much more subtle than the Outboard Power Supply mod, but I think all would hear the benefit.
It further pushed the T55C past the reach of the T25. I don't think the T25 can be coerced to catch up this this wonderful sound...
It's so nice, I don't wanna go to work anymore, I just want to relish in the lushness of this fine sonic rain forrest!!!