Definitley worth looking at importing it yourself and maybe paying some import duty - I'm sure it will still be cheaper.
The standard Tremec 3550 and TKO have 0.68 overdrive. I believe its the "TKO 2" that has the 0.8 5th gear. Better topend acceleration for road/circuit racing when 200mph with the 0.68 is a little high!
According to my Tremec service manual (2.6 meg PDF file) it says the TR-3550 is good for 350ft/lbs of torque and the TKO can take 375. I've read other places that say the can handle heaps more than this. The TKO is definitely stronger. The TR-3550 was US$100 cheaper so getting the TKO was a done deal. If you're interested in getting a Tremec from the US, I can give you the details of the Mustang/Ford speed shop that I mail ordered mine thru when I was there. Even in the US there's a BIG difference in prices people charge for them. Out of about ten suppliers I called the cheapest TKO was US$1350, others were ~US$1600. The 3550 is still a strong box though.
My XD is running BIG 295/50x15's. When I put my cheescutter 205/50x15's on I can feel a huge difference in RPM for the same speed. That's a 26.6" diameter vs 23.1. You can see my car pics in the Gallery under TruBlu351 user name.
Here's a cool driveline/tyre size calculation website.