It's not all about being quick but having a skill set you can count on when the unexpected happens.
I agree-ride within your abilities. However, if those abilities don't continue to expand it's much more likely for a rider to react incorrect when the right response is needed.
As a professional diver for 10 years we used to say to our classes for advanced diving or instructor training "do you have 5/10/15 (whatever) years experience or do you have 1 year experience 154 times over. Not to be cute but this is a very real possibility. The experience of not being able to stay with the H-D's in the turns suggests that the rider may not fully grasp the concepts of correct lines, proper turn in, delayed apex for the street, and many other techniques.
Then again, it may just be a casual pace. In any case, ask the experience question and answer honestly.
Proof this is valid-single vehicle crashes have overtaken the left turn accident as the #1 cause of motorcycle accidents. In other words, the rider has done it to themselves. And of those the great majority happen in the curves suggesting the cyclist lacks cornering skill.
As always, YMMV and this my $.02.