I have a carbon fibre tiller (off a racing yacht) and the clear coat/lacquer is cloudy, chipped and flaking in places.  In sanding it down with P800 is appears to be very thin and in places the surface become rough.  I suspect I am through to the carbon fibre because when I wet it it stays dull. Should I strip off all the clear coat and give it a coat or two of XCR and if so will it still require a clear coat on top of that?

i haven't much experience with carbon fiber or fiberglass for that matter but I have done auto body and paint more than 37 years, when I do come down to bare fibers I spray some BullDog adhesive promoter, come in gallon, or spray can from automotive paint supply store, maybe auto parts store, it seem to work good for me, 
i have never hear of this  XCR before, but reading about it now it sounds like you can just spray it on  and is very UV resistant so I suspect that it needs no other coatings...
If the clear is cloudy then yes you would need to get the cloudy clear off, as for if the fiberglass is dull looking when wet whit water I would be skeptical abouy that but the only thing you can do is try some XCR in small inconspicuousness place and see if it brings it out of it I think it would. here is the link about ithttps://system.eu2.netsuite.com/core/media/media.nl?id=29347&c=3937524&h=88e9f2a89dc17acb95bd&_xt=.pdf