When I changed from 512Mb to 1Gb I did notice a significant speed up, not double but say about 1.5 times. The way things are going with broadband speeds I think £15 is a bit steep as BT for instance is upgrading its customers from 512Kb to 1Mb and from 1Mb t0 2Mb for free. Unless you are very keen for extra speed I would hang on until Pipex succumb to market forces.
I have just gone up to 2 mbps with Bt and I cant say that there is much difference between this and my first 500kbps BB. Maybe if I was downloading lots of stuff it would be noticeable. I do find sending photo files much quicker though.