Guys , Phil takes a strong opinion on this issue , to which I dont subscribe, nor do i wish to discuss again here
As a rough guide
relatively its 30-40 bhp peak difference. (all other factors being considered)
Some curves on Awesomes or JKM's dyno pages for you to review (dont get hung up on figures look at the graph shape)
std K03 and std K04 gti have very different power delivery 'feel' , lag is slightly worse on K04
(and prob enhances the turbo spool 'push' wow factor)
K03 is seamless delivery from very low rpm.
The K04 ultimately has higher bhp available to it, both because it started out higher , and because the hardware has what appears to be a higher 'headroom' level for tuning before any shortcomings are hit.
fwiw : TopCats REVO stg2 K03 is just as quick as my stg1 ED30 , they 'feel' very different but the figures suggest they are very close.
both are fantastic cars and dont assume the K04 is necessarily 'better' (but you will want an ED30/Pirelli sometime soon
)