Hi Nick,
check out this thread:
http://www.golfgtiforum.co.uk/index.php?topic=56110.0There is a post from jonny which has a link to a site to check how changing tyre size will affect speedometer speed readout.
Basically, if you dont affect the overall rolling radius profile of your tyres you wont significantly affect your speedo reading wrong.
I dont know for Mk5s as I have a Mk3 (and I might stand corrected ) but you might be able to put 19" alloys on a Mk5 with low profile tyres. Overall radius will then be the same (touch wood) as 15" alloys with large profile tyres.
You wont have to change your springs etc unless you want to drop the height of the car (personal choice).
Finally, if you do buy new alloys check the alloy width will fit. I think it will be about ~8" wide that you can do without having arch work.