I lived in Rotovegas for awhile so I know all about the Kaituna, Wairoa etc. and I've been to the South Island for two month long trips and been on the Whitcomb and the Perth. First off, the jump from the Rotorua runs to the west coast runs is BIG. It doesn't matter what boat your paddling you better have the skills to run remote, inaccessible and sustained chunky class V to paddle the runs you listed. Those definitely are not the runs to cut your West Coast teeth on. Start with the lower Perth, lower Hokitika, and Arahura.
That being said the Stomper will take you there in terms of a great boat for the West Coast and you are correct you definitely want the 90. The west coast "creeks" tend be higher volume and more pushy than you'd expect but then they tend to still have really creeky moves through huge thundering boulder gardens. It's incredible paddling but not to be taken lightly. Like they say in BC, it's Strong Water. The Stomper is an easy boat to paddle in just about any conditions but I'd say excels in higher volume creek runs because of how easy it is to boof, how it deals with rocks, it's great initial and secondary stability, while still having great speed for it's length, a solid ability to carve and hold a line, and the "land and leave" technology, which basically means when you land a drop it likes to propel forward away from the hole. It's a great boat.
Cub boater: "What do the spiders mean?" Old fart boater: "Trust your intuition." CRCII