it all depends on what you are going to do. if you are going to be runing more boost then you will need to get a higher hear range possibly. your 6 runs ngk hear range 7 i think falkland have got a set in stock. if you are going to run slightly more boost you may need to gap them slightly less than standard. i know my 7 had to run 0.6mm or other wise suffered from a lot of hesitations on vacuum but that is because the 7 runs more boost.
i am running ngk hear range 8 which is what the extreme runs gaped at 0.6mm.
there is no straight answer it all depends on what you are wanting to do. the hks plugs are actually made by denso for hks.
if you want you can email me off line or phone me mate