An external wastegate is just the same as an internal one, except instead of being in the turbo exhaust housing, it sits on the end of a 5th pipe on the exhaust manifold. Therefore there is no longer a flap inside the turbo, and no longer an actuator attached to the turbo itself.

To do this on a RST would be quite expensive, and show no real benefit. You would need a bespoke manifold made up, plus a new popett style wastegate (Audis, Porsches have these). From the new wastegate you'd need it joining back into the exhaust, or stright to the atmos (the locals will not be pleased!) down a 'screamer pipe'!

TBH you'd be better off ditching the RST exhaust housing and elbow, and fitting a Cossie/Lotus style housing. I have adaptors which allow you to do this without any cutting/grinding or other malarkey. The benefit is a much better flowing exhaust housing, which is not prone to cracking, and is readily available in a range of sizes.

I posted a thread about the stuff I produce - have a search or PM me for some pics.

check google with 'popett' for pics and further info.