Firstly, check they will still sell to the UK, although it says they will, HD has directed them not to.
viewtopic.php?f=7&t=11334Secondly, I was considering something similar, but in the end I went down the route of XB heads for a number of reasons.
viewtopic.php?f=8&t=8142IIRC I went from about 80bhp to about 95bhp. It is not the horsepower increase that is most striking, but the attention to detail of carb and ignition setup that has given a really smooth and tractable engine from very low down. You can get the same by copying my setup.
When that kit arrives, it will be costing you £1100. If you went down the XB route, which would be much cheaper, you would save enough money for some great stuff such as (and I really believe you will save enough money for all these things):
1. Spondon440 front mount (which you would need cos the front mount holes are closer together)
2. Mikuni carb (which makes a big difference in throttle response)
3. Dyna2000 or Screaming Eagle or similar programmable ignition unit (will give you a lot more bottom end - just right for overtaking)
4. Cometic gaskets to get the squish to a minimum
5. Forcewinder
6. Set of new tyres
7. Set of headraces
8. Set of wheel bearings
I recently did a back to back against a 7000 mile S1W which has the Thunderstorm heads, race ignition, race airbox, CV carb with Thunderslide kit, and it felt flat in comparison to The Grenade. In reality it is probably a good example of a largely standard S1W.