"6" heat range should be fine, considering the factory spec is a "5".
The gap has no effect on the "speed" of ignition, so the timing will not be affected to any significant degree. The type of plug is pretty much irrelevant. Pt plugs are used standard because they require less maintenance and a longer replacement interval than Cu plugs. In all other respects, they are the same - they simply provide a facility to ignite the fuel / air mixture inside the combustion chamber.
Try opening the gap out to, say, 1mm. This should provide a stronger spark, and therefore a better burn of the fuel/air mixture.
Or maybe you are just used to the power delivery, and there is no problem at all.