Nicky, interesting one that. If the water is clear and its anytime between spring and autumn, I would use a floating line with a LARGE fly. believe me, the Pike will come up for it. It will still be fishing a few feet below the surface, so dont worry too much. One tactic I use is to let the fly sink to the bottom...and then strip it in fast...it looks like a wee fish darting towards the surface and Pike just love it.
The only time I now use a fast sink line is when fishing from a drifting boat and i want the fly to sink fast before the boat drifts over it. Its a tactic the trout anglers use.
If the water is dirty, I use a neutral line or a slow sink. This way i can fish the flies slowly along the bottom. if u use a fast sink line in this situation, you will be hitting bottom too much. Its possible to use a floating fly on a very short leader and a fast sink line....but thats rather specialised.
Hope this helps