Hi Matey.
Can i suggest you try using 3 new springs & 3 old springs in a Triangle formation, this will give you more spring pressure than 6 old springs but won't be as heavy as 6 new heavy duty springs... (An old racing trick) to find enough spring pressure without clutch slip at high RPM with the bonus of a lighter clutch lever feel.
Regarding drilling the basket for better oil flow, this is a free Mod that actually works wonders on any wet multi-plate clutch.
Personally I drill all my clutch baskets, I've found it works well if you get a marker pen & mark the location of the steels on the basket, then drill 3mm holes in these areas, I stagger my drilling in a spiral type pattern & slightly countersink all holes drilled, this massively improves oil flow throughout the plates & I've yet to see a dry plate after this Mod.
Hope this suggestion helps?