I have the same cooler as Chiptoothy it seems. It is a 125ml capacity cooler, and with the volume of the lines included it takes about 250-300ml of oil on top of the roughly 1.2-ish quarts the bike already takes. Basically adding about 25% more capacity to the system. I mounted mine up about as high as it can go, both on the bike physically, and the limit of the cooler lines as well.
The fender previously being spaced down at the rear mounts about 5mm bought it just enough room for the cooler to fit there. Yes, the fender is going to block some airflow to the cooler, but enough of the core gets airflow as it is that it doesn't harm it's ability to cool. I do have a fender I plan to buy in the future that has slots in that part of the fender that should aid with airflow there if I decide it needs it. Even with 50 degree air temps that cooler gets quite warm. A big advantage of this cooler location is that it doesn't block any airflow to the engine itself.
I also noticed my idle get much more consistent. I can only assume my cylinder head temps are more stable as well. If you plan on bumping up the compression then I would definitely recommend some type of oil cooling. At least the valve cover style.