12427.4 miles is quite a lot. Personally if the car is a daily driver, I'd be changing plugs every 3 months.

I'm sure Hanyes manual says to change plugs every 3,000 miles on RS turbos.

bcpr8es NGK plugs for me, for the price of plugs is a no brainer to swap for new plugs regularly, Wire wheeling the plugs using a drill bit during 'Sunday' maintenance never hurts either.