There are really no parts from the Focus ST that translate to the Fiesta ST except maybe the limited slip diff might be compatible.
The Focus RS Revo-knuckles I mentioned would be essentially a complete redesign of the front suspension and possibly steering, an extreme solution if I wind up with a really bad torque steer problem.
I don't think it will be bad, something like a Cobb stage 3 Focus ST has lots more torque than I can hope for and I haven't heard people saying the torque steer is unbearable. The Fiesta ST is just a slightly smaller and lighter car, there is no reason to expect big problems from around 300 HP.
If I'm going to upgrade the turbo, the EFR I'm looking at is the option that should spool as quick as anything bigger than a KP39 and once you get to the point of a new turbo, new plumbing, ECU tuning devices and software, it does not make sense to leave power on the table if all you have to do is write a decent tune to get it. Its not a matter of building for 300 HP, its a matter of building for that turbo and 300+ HP is the potential if you build it right. If there was another turbo out there that would spool quicker and still give me more high rpm boost than stock, I would be trying to figure out its match but there isn't anything like that out there.
I'm going to try to do better than a decent tune because I know some people who can help me squeeze out more. A race tune with ALS might be possible so I'm going to see what can be done. I would probably use ALS with a two-step rev limiter and neither would be used often, just a tool for certain race situations when points are on the line.
What makes a turbo spool quickly:
#1, low inertia in the turbine and compressor wheel.
#2, low friction in the bearing system.
#3, a twin scroll housing to better scavenge the exhaust pulse energy at low rpm.
The turbo I'm looking at is the smallest EFR turbo and has a Titanium Aluminide turbine wheel which has about 1/2 the density of a Nickel super alloy wheel (like Inconel) so the inertia is low. The bearing assembly uses large ceramic ball bearings rather than plain bearings, this reduces friction and increases the maximum bearing thrust load which improves the durability. It also has a twin scroll housing so all the boxes are checked.
If you want more power than bolt-ons can provide, its going to be this path or very similar. Its a shame but the KP39 is just tiny so the normal bolt-on stuff will only give small power increases.
I'm not worried about changing the balance of the car and I'm not going for a drag race setup, if anything I'm going for a time attack/street setup.