Steve, changing the differential will give a different final drive ratio. However this affects performance in more than one way. Going to a higher ratio will effectively raise all four gears equally, which of course lowers the RPMs at a given speed. So driving on the highway at 55 MPH will be much more pleasant - less noisy and easier on the car. However the downside of this is that acceleration will suffer. The TF does not have a lot of horsepower, so raising the gears will put additional load on the engine during acceleration. It won't hurt any, just slow down acceleration a little bit.
You can equate it to changing gears on a bicycle. Changing the rear end in your car is like shifting into the next higher gear on the bike. Your pedaling slows, but it's harder to accelerate, making high-speeds more comfortable but lower speeds less so.
There are a few different ratios to choose from and I don't remember what ratio the TF came with originally. It's probably a 4.55:1. You can upgrade it to 4.3:1 which should give a good balance between lowered RPM and minimal loss of acceleration. I'd recommend talking to a local TF owner to see what his car is like, as most likely he has changed the gears. Good luck!
PS... FWIW, My '53 YB revs very high on the highway and i do plan to change the gearing to better suit modern traffic. It was geared with slow, windy British roads in mind and can pull a tree stump in first!