Thanks for all the help here, and I'd just like to add that I presumed correct but wanted confirmation because I was getting head bogging at quite mild settings. It made me suspect I was doing the checks incorrectly.

As it turned out (and I sort of suspected), the problem was mainly down to an old muffler choking my engine's top end. Just fitted a Hatori 3D2 muffler (that I had been impatiently waiting for) and it now feels like I could run plus/minus 20 deg colletive and another 30 degrees of cyclic (just kidding, but you get the picture eh??) This thing rocks, together with the YS50!!