Isn't the ProDrive aut tranny the one the Knight Racing used in their RS-Ts for the 2001 North American rally series? I was talking to one of them about it and I seem to remember that their trannies cost about $12-15k. They never had a tranny break on their cars and they were running close to 450 lb/ft of torque out of those turbo EJ25s.
Beside that there is also an STi 6-speed box that's a little stronger but those still go for $8000. I spoke with Gettrag about custom building an indestructable tranny for my car and they said they'd do it for about $10k. There's a new gearset that someone had advertised for about $3000 that didn't look to bad, would very likely work in an EJ20T. There is also the STi ver5 gearbox which is what most of the independent Subaru rally cars use in other parts of the world. I think those go for $2500 and sacrifice a bit of top end for a lot more acceleration.