I’m not sure about your AP Racing kit. I haven’t seen their kit on a Fiesta here but a lot of other cars run them. I looked when I was upgrading and didn’t have much luck. The Stoptech ST40 is plenty of brake for sure but likely heavier than the AP. Consider upgrading the rear as well. I had a lot of bias before I upgraded the rear to the Focus SVT option. You can find it at Goldcoast here: http://goldcoastautomotive.com/cgi-bin/p/awtp-product-category.cgi?d=gold-coast-automotive&pc=16399
There is one other chap doing the exact conversion I'm after here: https://www.fiestastforum.com/threads/ap-racing-cp9200-315mm-big-brake-kit.21985/page-3#post-406264
So waiting to see his outcome, but annoyingly his wheels are delayed due to pandemic.
I have seen rear upgrades from older models, but hadn't really considered it. What behaviour did you witness with just fronts upgraded?
I'm not doing timed stuff, just fun laps with mates. So don't mind being a bit slower, but obviously stock brakes are just heat soaking almost after a few laps.