Firstly, I'm probably the least technical person on this forum - by a mile, so all my views are based on my experiences during ownership and those of many other F drivers (who I meet on a regular basis).
With regards to the radiator, all I can say is that one member had his replaced - can't recall why, but Lexus Japan didn't even have any in stock. It had to be made!!!
I'd be careful assuming that parts for the IS-F are easily swappable with IS parts. As an example, my IS-F had a leak from the air conditioning system. Replacement pipe was IS-F specific. Time to fix, several months as the part had to be ordered from Japan. Who'd have thought the pipework wasn't just off an IS (or other Lexus)!!
In summary, unlike the world of BMW, Mercedes, Audi, etc, there simply isn't an abundance of independent garages with knowledge of the IS-F, so when things go wrong, most of us will have to resort back to using the main dealers. This will never be cheap. Of course there are those talented individuals that will engineer fixes, make their own radiators in their back shed, rob pipework from the washing machine to fix the air con etc etc, but whichever way you look at it, this is a pain.