Hi mate, Colder thermostat makes no difference once you are over the rated temp. Once it's open, it's open. Colder thermostat delays this slightly though but in my experience at a track day, the engine block heat soak never allows temps to get back to under 80-85 without a 2 hour break. (Ambient dependant)
To answer your questions:
I've used 2 X 55mm PWR radiators, 1 X Fenix, 1 X Chinese ebay radiator and 2 stock. PWR seems to be the best but 1st one died due to stray current (possibly poor installation or crash causing rubbing: metal/metal) PWR suck with warranty. Fenix are awesome with warranty but don't make for FG anymore. BF ok though. Check end bracket fittings though as quality varies. My second PWR 55mm V8 one is going fine but very close to belt/fans. Works better shrouded in heat proof foam and spacer at back of bonnet. Manual thermos fan over ride switch is currently working for me keeping coolant temps under 85deg in staging pit area.
Re: Engine oil coolers. Highly recommended. Mine sits behind fog light with a thermos fan pulling air through. Switch on manually usually on 2nd session of a track day. More complicated relay versions are available that turn on automatically at a preset temp but I like to keep it reliable and simple.
Redline water wetter and Purple Ice didn't really do much.
The real secret is..................go in winter.
Take a big floor mounted fan and drive about 5kms up the road at low rpm 100kmh after a session for adequate cool down.
I'll be at Wakefield with a few other XR6T May 30th with Trackschool. Can you make it?
Feel free to PM me too.