Stick with the DCPR8EIX. Stock is an 8 heat range from NGK.(Different brands use different heat ranges, lower means hotter with NGK) I had fouling issues with trying the colder plug so I don't recommend it much to the disagreement of many on the forum which Im not sure why. You're talking a 70-100°C difference with each step. You want to stay in the self cleaning zone. If adding a turbo or increasing compression Id agree to go colder.
FYI the Denso 27 is equivalent to a 9 NGK
The 10R12 is equivalent to an 8 NGK
You may be overly rich or have a weak coil/bad plug wires if you are fouling everything that quickly though. See article linked below
http://www.ngksparkplugs.com/about-n...rk-plug-basics