IMHO oil cooler a must if you track the car. On mine aside from oil cooler i have also put vents in the wing splash shields and fitted the vented aluminium undershield to allow hot air out of engine bay now only see about 120/130 when tracking.
I checked my invoices and the cost was actually £10,600 in total but i fail see how the GTm system could be done for less than £4000 as it costs almost £7k to import from USA with VAT + duty.
Actually could have been S28 only spotted the car quickly in passing in opposite direction while the wife was talking, so hello to you Arran, car is very clean and shiny