I ride a police ST1100, and yes they get hot idling and in slow traffic. Taking out the thermostats wont fix the prooblem as it just means hot water passes through the radiator to fast to cool properly.
Check that the thermo fan is coming on when the temp climbs, some police bikes had a mod done, so that when the bike was parked with the strobes on, the anti theft switch also turned on the fan to keep the engine cooler.
I dont think that anyone will know the thread size, as no-one will have thought about an oil cooler.
A quick google of the oil filter spec shows :- 13/16 - 16 UN-2B