Are you changing the Head Unit?
As many have pointed out here, the standard is not too bad. I did test drive a 147 with the bose setup and it had a tad more bass , but wasn't what I'd consider qualitatively better.
I wanted to change the head unit anyway as I wanted to have a unit I could connect a HDD to and keep all my tracks on there.
I found a set of 4 BOSE speakers on ebay for £20 and upgraded the door/rear units with those. as a plug 'n' lay replacement without upgrading the loom. This way you get the improved combi drivers in the door but retain the high level tweeters. Because I'm replacing the head unit with one that is happy to drive 2 ohms it's no problem.
Then added a 'juice' 12" sub in the boot to handle the bottom end. You can buy connector blocks so it's easily removed when you need to fill the boot with shopping or bikes ;-)
The important bit is that your head unit should be able to apply band-pass filters/eq dependently to front, rear and sub speakers. There's little point adding a sub if you're still going to send all the bottom end to the door speakers too! By cutting from around 100hz to the doors then there's enough juice in a fairly modest 50Wx4 head unit to get pretty decent sound out of the Bose's then offload all the heavy duty stuff to the sub. Overall it sounds great without having to splash out too much. Ok with the sub out it sounds a little tinny, but that's a compromise I'm happy to live with.