yes most are using an external wastegaste with a huge manifold ( usually custom made)
here's an GSi running the GT30.
http://www.astraownersclub.com/vb/sh...d.php?t=225252 might gve you some ideas on the manifold and external wastegate setup
the GT35 is rated to 770bhp! mad. There is a guy with a 500 bhp zafira gsi running the GT35, it generates so much heat he is having to run 2 massive radiators and no thermostats. The temp gauge goes from stone cold to hot just blipping the throttle.