Could be the engines are starting to loosen up too as well as the new tune to suite the earlier timing.
I too used to run OS and RB plugs but are too expensive here in Aus compared to the OD and I didn't find much change in performance within the same heat range, most of us run medium heat range as a rule but I too have run a hotter and a colder range when in a different state.
The key is to find what works for you and where you race.