The number is the heat range the plug operates at, 7 being standard grade iirc. With the NGK range of plugs, the lower the number, the hotter the plug. This means that a heat range 6 plug will be too hot for even a GTiR running standard boost !
Stick with the 7 if you're running standard boost, if you're running 1 bar, spec an 8 range plug. Or better still, go for the Denso Iridium range of plugs. IK22 for standard boost and IK24 if you're running 1 bar !
For info, have a look here:
www.sparkplugs.co.uk