I've done zero upgrades to the performance so far,
My next purchasing steps are
1. Rear motor Mount
2. Short throw clutch kit
3. I really want a fuel 4 port aux line to help keep my valves clean
I would like this car for a long time- and those poor valves get neglected daily cleaning.
Just purchased the car at 22k miles,
The only things I've touched on it so far is
-mudflaps
-pulled the intake manifold off to try to get some kind of cleaning done/ and to see what those valves look like.
I was thinking of going single aux for the valve cleaning as well. However if you look around here the whole valve cleaning theory is pretty controversial. I think most people agree that aux fuel will keep your valves from getting worse but it probably doesn't reverse any build up already there. To be objective, no one has actually done a scientific study on aux fuel and valves on the Fiesta ST. Everything is anecdotal.
What *is* conclusive is tuning for aux fuel is tricky, the ECU isn't designed for port aux line so now you're adding in a line of failure to "fix" something that isn't a line of failure. That was my thinking anyway. Many tuners recommend going uprated injectors if you want more power and HPFP if you want even more power. As for cleaning your valves, walnut shell blasting the valves cleans them and does it well and safely. You can read around here how certain BMW techs were trained to provide this service because of early BMW DI engines would get dirty valves and you can pay one to do it anywhere between $200-500.
So you can pay $1500 for an aux 4 port. Get good at learning to wire your car to power the aux port. And then another $500 for a tuner to tune it. Or you can just take your car to someone trained to walnut shell blast your valves every 50-60K miles for $400 a pop and probably save yourself a lot of coin.
Oil catch can also theoretically keeps your valves from getting as dirty as quick too.