It won't be too loud especially if you have the resonated cat-back so don't worry. If you'd rather have a sports-cat though then hold out for one or buy a 3" downpipe & sports-cat now. You can then save up £300 for a decent FMIC at another point and go for stage 2. Alternately go for stiffer suspension & ARBs or ARBs, engine mounts and polybushes to get the traction down better.
Believe it or not £600 is a fair amount to spend on mods for this car so you can potentially get some major parts sorted now.
Consider an LCR brake uprade perhaps or 312mm upgrade? It's totally up to you which way you go.
think your spot on, there is so much i could do with the money that i just cant decide. brake upgrade is a good shout, but sstems back to wanting bigger wheels and suspension. Now the sale section is closed, bargains are hard to find