Yeah changign them every 20K doesnt seem to bad to me. I may have to do the same thing. Run the AR103 in the summer and a different one in the winter, maybe the 104.
Changing them when i change my tires just to check the plugs and how they are burning probobly wouldnt hurt.
Also with how over priced everything for these cars are $15. for a set of spark plugs is a bargin in my book. I mean $200. for a CAI? A bent tube and a cone shaped air filter does not cost $200. but we pay it.
$1200. for wheels and tires, $1000. for a 2" drop? Im good for $15. plugs every 20K