im not sure about the atx thats from an spi model car... but the atx from the zetec based car will hold up to 150-160whp (from personal expereince) on a day to day basis aslong as the fluid is kept changed (the use of an aftermarket tranny cooler is stronly suggested)
the stock atx behind the zetec will not hold up in stock form with any type of forced induction... so if you want power... the tranny is going to need attention whether you swap it for a manual with a highperformance clutch.... or you swap in a built automatic from a company such as lentech... swapping in an automatic from a different model wont effect power handling by much... and if your wanting 300+whp... i dont believe there is any option as far as a "stock" automatic goes that will swap into the focus and hold up to those power levels... lentech transmissions will be the only way to go.
do this... dont worry about power and trannies and all that for the time being... focus (no pun intended) on getting the car to handle well first... complete the suspension setup and get some decent rubber under it... then brakes... then start researching the power side of things... if your wanting 300+whp.. your looking at spending $6k+ on the motor and trans.. proabably closer to the $10k range depending on what you want to do... but a 300whp focus that sticks to the pavement is hard to beat...
to put it into perspective... ive got roughly $22k invested in my car including purchase price which was 7k... so $15k in performance and aftermarket parts... which it isnt finished yet... to finish it up... i'll have closer to $17k in aftermarket parts total to have it where i want it to be.... a daily driven 300+whp car that can out handle most factory higher end sports cars... so basically since 99% of the car is new or has been replaced within the last 5k miles... ive got the car I want basically new (exept the shell) for a total investment of around $24k that can out-run and out handle any factory car in that price range.. and with a power to weight ratio better then some corvettes... you get the picture... but the look on the guy in the corvettes face when you blow by them at the track... priceless

(considering their investment to purchase that stock car was more then my investment to build the car that is passing them... and the fact that its a 4cyl focus just adds insult to injury)
anyways... back on subject... suspension, brakes, and tires first... get that lined out and worry about power later... you'll be amazed at what a good handling car can do on the track compared to a high whp car that cant keep the tires stuck to the pavement.
-AcId-