They don't make peak bhp on std cams till around 6400, so going to rev to around 6700 at least.
Is it the same rpm in each gear it is limited to ?, DON'T work the engine hard(boost) until this is sorted in case it is a fuelling issue, could leave you with a dead engine!!
If it runs perfect up to 4500 then it's not the rotor arm, what gap are you using on the plugs ?, they work best around 0.6mm, not the 1.0mm ford state, mine wouldn't even start with that gap!!
When was the fuel filter last changed, if not within the last 2 years change that anyway, have you rewired the fuel pump ?, if not do that, needs doing on any cossie even std power ones, as well as the fan rewire.
tabetha