I wouldn't go with a hotter plug as all this means is that the plug will not dissipate heat to the head as fast and thus the plug will run hotter. If it runs too hot it can cause engine damage. I doubt going to a BR8ES will go that far, but I still wouldn't do it.
I would however look at your oil pump setting (make sure the cable housing it back together), and your jetting. It does sound like you either have too much oil or you are too rich.
I don't have my tech info here with me, but if my memory serves me correctly Yamaha has still not put a thermostat on the 600 triple engine. This does not allow the engine to warm up unless the engine gets worked a little. You may consider adding a thermostat or a heater depending on what climate you ride in.