Chris has got it right.
If i were you Peter, there is no harm in fitting the stat. AlanG always used to say to drill your OE stat, but for all the cost of a Zerosports, it isn't money thrown away IMHO.
Oil and coolant work together in keeping the overall temps lower. Oil temp above 120 degrees is getting serious, but i do know of people getting to this temp and not bothering with an oil cooler....and it is their opinion and not just about not caring. These temps, you won't get on the road. Too many things have you going slow, ie: other road users, roads you don't know well enough, weather conditions and the missus.
I used to see 115 degrees oil temps with the 2.5 quite easily. Since i fitted the new turbo and thermostat, i have seen an absolute max of 105 and that was including the day of mapping.
The stat has dropped the coolant temp by circa 10 degrees overall too.