Righty then!!!!
Precat removal is a simple enough task. Remove the downpipe away from the manifold and the catback. Don't forget to unclip the sensors in the downpipe. Use a vice to hold the downpipe and remove the precat with a lump hammer and a cold chisel. You can also make a start on it by drilling some of it out. Remember to tip the downpipe upside down when you have finished to get any of the crap out of the downpipe before refitting. Leaving particals of precat to get blocked in the main cat is bad. Make sure that you order yourself a new gasket to fit back on between the downpipe flange and manifold.
You say that you have a Powerflow exhaust. What diameter? Was it just a normal 2 inch replacement in stainless or have you gone upto 2.5 inch. A 2.5 inch catback system will give you more power and torque especially if its coupled together to a 2.5 inch custom stainless downpipe. Also a 200 cell sports cat will help things flow better.
Its worth upgrading your intake side of things too. The dbilas above would be a good addition. Other than that, a larger bore custom aluminium intake tube with silicone elbows will do the job especially if you've already spent money on the Viper.
Only after the above has been done would it be worth having a remap. You would be able to make the most from your mods then. If you intended on going further with head and cam work then it would be advisable to wait.
See this link for custom downpipes and 2.5 inch catback systems from Piper for the Astra G. Well worth spending a bit of money I think.
Scroll halfway down the page once its loaded up.
http://www.courtenaysport.co.uk/
;-)