theres a couple of options;
Autotech and KMD do a kit which uprates the internals of your std pump, think these are just over 300quid fitted, your pump needs stripping and the parts fitting
APR do a complete replacement uprated pump for just over 700quid, or they do an exchange 1 where you give them your pump and you get an uprated 1 for 580quid I think
I'm looking into this myself at the minute for the S3, had the APR one on my ED30 and it made a big difference to the midrange