Matt,
The C4 in karens car is worth a lot more $$ than the TKO600 in mine.
The C4 will hold more torque, more power and the shifts are rediculously quick.
We are fitting one of the overdrives,
http://www.gearvendors.com/hrford3s.html
And that will give it a good legs on the freeway.
I am suggesting that an AOD or tricked up C4 would be an upgrade, but yes you are right once installed the running cost would be less than a manual.
Phil