Well in the new Impreza mag that just came out theres an article on the new STI 6spd M/T. They show pictures of the inside of the transmission, the gears and even compare the gears side by side with the old 5spd M/T. I'm working on getting a translated version of the article but I thought I would at least post up the pics here since people have been talking about it so much.

 

Here is the side by side comparison of the gears. Notice how the new shaft is a larger diameter as well as longer. The distance between the gears has changed because of the use of new shift forks and linkage. The best part is that they made the gears for 1st and 2nd gear larger and more like the 3rd and 4th gears. As well as making them thicker to give it more contact area to spread out pressure. 2nd gear is wear the difference is very noticeable. Its now about twice the size of the old gear.

 

They also changed is so that the case is no longer one piece going from the bell housing back to the transfercase.

 

Here is also a pic of the rear hubs. They are definately larger and would need to be replaced when going to the 6spd.

 

Even with all the time and effort put into the new STI setup they still try to save a buck by going with the old plastic end links for the rear.