Agree with Rider1150 - I have just converted my low beam to 35W HID's with a kit from eBay that was imported from the states - £60. I bought the 35W instead of the 55W and so the results are that good I can't see the real benefit from the 55's - it's early days yet though.
The price hike from the 35W to 55W is quite a bit too.
They were also very easy to fit, even for me!
The one thing with the F800 is that it is CAMBUS and so you have to make sure you get the right kit. Go to a good retailer and they should point you in the right direction.
A friend has just done the same to his Daytona 675 and the results are equally good and immediately obvious.
In summary then, the additional light from the 35's is perfect for those occasional night excursions. Highly recommended over the additional and expensive spots.