Hello,
After some basic default tweaking, and then spending about an hour on the setup, I still get a lot of fish tailing / backend movement from the XF GTI.

The car really wants to swing out when I brake into corners, and often when I'm just coasting.

Does anyone have any suggestions on how to solve this?
I would appreciate any help at all.
Thank you!

Perhaps the tyre pressure is too high? Try lowering it a bit or adding a bit of negative camber.

Also people usually use hybrids instead of the road tyres as rears on BL1(no idea of other sets for other tracks). If you want to take the easier route, go to SetupGrid or SetupField and download a setup from there.

Quote from nullsys :

The car really wants to swing out when I brake into corners, and often when I'm just coasting.

i've always felt that FWD cars do that no matter what. perhaps your brake bias is too far back. try 70%. as for when it's coasting, maybe you're turning the wheel too far?

Lower tyre pressure is a bit of a final thing,

Move your brake balance to about 75% front...

IMO, wouldn't the suspension of the car determine the optimal brake bias?
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I'd probably say increase some front sway bar to decrease the overall oversteering tendency. For XFG, make sure that you have good tyre compounds (ie. Norms front, Hybrids back).