I know that at higher revs the valves on a standard engine start to kiss the pistons. I've currently got the soft cut rev limiter set to 6800 but the power is still climbing steeply, so what needs doing to raise the limit up further?

Is it a case of solid tappets and stiffer valve springs or are valve cutouts in the pistons needed as well? The bottom end has had a full dynamic ballance so AIUI that should be good to at least 8k rpm and the head has had a good fettle so there's every reason to believe that there's plenty of top end there for the taking.