Ive read now, a few times, that GRP plugs are the best for GRP engine because they are specifically made for GRP turbo engines, so they have a perfect fit. I think its just a matter of finding the right heat range(plug) for the fuel and conditions your running in.

Im not %100 sure about this but OS plugs are made for not only their land running engines but also their airplane and boat engines also, so i dont think they would be as a good fit for GRP turbos.

I realize the GRPs are twice as expensive but ive read also a few times now that they last a hella long time, i think OS plugs are fine to use if you are stuck without a GRP plug though.