If set up correctly the bike should be good for 100mph even after almost 30 years. Its book speed when new is a 105mph.
You have to hold the gears and not short shift, arrive in 5th gear at 60 and you will never get to 100mph.
Having said that, being caught doing 30mph over the limit in the uk can lose you you license in one hit, so is not recomended.
I find my bike very stable at all speeds it is capable of - unlike some. I have clip-ons and rear sets which make the bike more comfortable at higher speeds (50 upwards) - but less comfortable below that.
The noise is glorious when tempered with ear plugs.
So the bike is capable of far more than you are asking of it. I find that 70-80 on the motorway is good, as I am then crusing along with the flow of the traffic, which is important for safety reasons.
On the A roads, well that is another story - sometimes slower, occasially faster - but not by much!
Steve H