I think the answer depends on the 'total modifications'.
If you are just installing a K&N, I doubt you will notice any significant difference in performance - just more intake noise, and you won't have to make any other changes.
If you are going for:
- New Headers (no cat)
- New 'free flowing' can
- K&N
- Power Controller
Then you will notice a significant difference in performance and 'usability', but the Power Controller is the 'icing in the cake' and wll adjust the mixture accordingly. I have this on mine, and it's still pulling hard at 180 to 190 km/h (112.5 to 119 mph) - this is not the main benefit. The main benefit is the MUCH better mid-range and the elimination of the 'flat-spot' around 5,000 rpm.
So, in essence, it is the suite of mods that makes the difference - if you're only going for the K&N, then I wouldn't bother (no doubt there will be many other opinions!).