No, not difficult at all. Took me about an hour and a half. I did give some tips on one of the precat removal threads about drilling the precat out, make sure you drill all the way through it and also it's worth using the biggest drill bit youve got to drill as much out of the centre as you can. Thenit should chisel out no probs and you can clean up the inside of the pipe with a grinder or dremel thingy or even a file.
An easy job and worth doing.
The Astra is a great car, there are quite a few mods to do without breaking the bank. The precat and air box mods are completely free, so you can get something for nothing then. Well done Vauxhall............