I've now realised who JamesH is (i'm a bit slow) and so i'm already aware of his problem!
My oil pressure is normally 45-50lb/ft when cold but doesn't drop below 20 even when stinking hot.
For my car there's a fairly distinct point when the oil gets too hot and starts dumping that heat into the water as the top hose temperature stays steady at 90ish for quite a while then shoots rapidly up to 105 and sometimes beyond.
I've just finished upgrading to a larger radiator and that seems to have improved my cooling by more than the area ratio led me to believe. Hopefully that will allow me to avoid refitting the oil cooler. I'm off to Donington on Friday night (£99 for 3 hours open pitlane!) so will give it a thrash there and see if the water still overheats. If it does then i suspect the oil temp will be the root cause and go back to the separate cooler.
Like you say, it's better to lose a little power than have the whole thing go up in smoke!