Stock suspension is fine, you'll go much quicker with the tyres. Been there, done that.
Chris at CG is amazing, he got my 911 working beautifully, but probably overkill for a relatively stock Zed. Still, if you have £300 spare it really is a bargain, even at that price. He'll be quite booked up this time of year, but if you're flexible he might be able to squeeze you in. Didn't realise he did Jap stuff too.