Brad Black has done all the hard work for you, just check out all his 851/888 reports here:
http://www.bikeboy.org/performance.html
A lot of it depends on how much you can do yourself. Cam timing costs virtually nothing once you've sourced adjustable pulleys and can do it yourself, but wouldn't be a cheap exercise at a workshop.
If you haven't already, have a read of the the GCR's, it could save you some heart/brain ache. Note they are constantly evolving. For example new for this year in P6: stroke is to remain standard, so no dumping a 900 crank in it for big cubes, and fuel must now be unleaded and available from your local servo.
Sometimes they even make it easier for you, until this year you had to have case protectors that were more than just glued/bonded on and it is an absolute nightmare trying to find a composite protectors that bolt/screw on to an 851, particularly the alternator side. I ended up making my own. But this year, and for Historics only, they have allowed bonding....but they stipulate exactly which bonding agent you must use. So a few minutes reading the rules can saves a lot of hassle.
