It's not the horsepower that breaks the EVO TC.. It's the torque. You can get a max of 350wtq before you're settin yourself for catastrophic failure. I've heard of some with 500+whp and 350wtq moving along just fine without slip. You have to also program the TCU engage the clutches more sharply so that it can handle the extra torque. :)

sounds like a lot of work, and expensive.

Not sure about that. From what I hear it's just a tune of the TCU along with the ECU. Which the expensive part is that there aren't many shops that know how to do this.

GST in California is the shop that comes to mind.