So I've been spending some time doing further research during my commute and I have come to a few conclusions to update my system design. The main change has been to add in an oil thermostat, as briefly mentioned early on in this thread by rainman.
Derale make an oil thermostat which is pretty affordable, link is here:
http://www.summitracing.com/parts/DER-13011/
It provides a bypass at 180'F (82'C), so this will allow a nice warmup via the heat exchanger in winter, and a 95% bypass at full open thermostat - straight to the external oil cooler. The best of both worlds, especially taking into account the arguments of line out temp vs sump temp.
There is also a bracket and hardware kit available here:
http://www.summitracing.com/parts/DER-25793/
Revised diagram attached for feedback below: