When does it do that exactly? If only when powering out of or through a corner, it could be the inside rear wheel slipping because your rear diff is not very good... i.e. a viscous diff or a plated diff that needs servicing.
The "clutch is poked because it engages right at the top" is an old school cable clutch thing I think... you have a hydraulic clutch.
The Sti does have an intake temp sensor and so will feel down on power when heat soaked... the WRX does not have one... true for v5/v6 in any case, but I imagine a v7 sti will have one.