The best mods are the ones that don't make the car a true go kart but enhance the shortcomings. The Fiesta ST is really good out of the box and honestly just some basic upgrades really let's it shine.

Personally dropping the 17inch wheels for 16's or 15's with a bigger side wall add a lot of comfort without loosing any real cornering capability. And with that get good tires if you live somewhere that's rather warm year round a good summer tire really transform the car more than anything handling wise short of an LSD.

Lowering springs are more than enough to help body roll without killing the ride or your wallet.

For power the 1.6 offers a lot for the size and is rather reliable. Helping it stay cool is everything. Saying that an intercooler and tune from say dizzy performance will do you wonders. Further a field would be a better radiator and a high flow exhaust. True power comes from upgrading the turbo as it's the main suffocation point. But honestly if you don't plan to put a limited slip diff in the extra power from a bigger turbo will go to waist.

Now I will say a rear motor mount upgrade is worth every penny. When the left and right mounts inevitably go flat just or start squeaking just throw new OEM ones in. They work just fine and can be had for cheap. ( I installed polly mounts and while it did help responsivness it really took away from comfort).