No mention of cam chain stretch that I have ever found. My bike has had an easier (if wetter) time than your bike Al and when I changed the cam chain at +90K miles I couldn't measure the stretch it was so small...
I've never replaced the head bolts or cam holder bolts, hmmm, probably should.Don't remember that in the manual. Also nowhere in the manual does it mention an amount for cam chain stretch.Please if someone has seen this let me know. I replaced mine last year (64,000 km & several hundred 1/4 mile passes) and it had stretched a leeetle bit. Maybe a couple mm.
I've never replaced the cam cap bolts on my bike, with a torque of 12Nm stretching them is never going to be an issue. I clean and inspect them at each valve check and have never seen an issue.
The one set of bolts that can/does suffer from stretching are the 'decorative' bolts that hold the cam cover on. If the history of the bike is unknown I would be checking these, the bolts are very soft.
Notice the stretch just below the lower collar at the start of the thread.