Ok, I have you lot to thank for this !!
Having read a few post's on here with regards to the above rpm, I decided on my 130 mile ride home from work that I test out your theory/ allegations.
Oh my bloody GOD(please excuse the use but it's the cleanest choice)
There I was just cruising along in 5th gear at 4000 rpm when my lane became clear ahead for a couple of miles and the thought just rushed in to hit the 4500 mark and go past it, so I did just that. I crept up slowly and along came the noisy vibration and rattle that the Dr Jeckel in me would shy away from but the Mr Hyde grabbed me by the scruff of the neck and told me, nay, SCREAMED at me to go for it...!!!!!!
4700 rpm, the vibration is dissappearing...
4800 rpm, smoothness and power seem to in abundance...
5000 rpm, I'm smugly grinning like a bloody Cheshire Cat...
5200 rpm and my space is running out
This is definately to be continued