The 15HP of Morini the Minx can be a bit asthmatic above 55 - 60, but the rasp of the exhaust is magic and thrumming along unclassified and B roads is just grand. Longer journeys (100+ miles can get tedious on A roads). My old AJS 18S would have fitted in this category, but was used for commuting between Hertfordshire and Cumbria up the M1 and M6. It was fine at a steady 60 - 65mph, but your mind had to be in neutral and the 280miles took a wee while!

Wee Duke 250, 350 are just good fun, as is the Honda Bros 400. All have a top wack around the 'ton' an put out 25 - 35 HP depending upon what you have and done to it. Wee Dukes are nice and light, but get kicked about on rough roads or 'drift' sideways in the wind. The Bros at 180 odd kg is more planted, but the suspension is 'limited'.
SRX 600, F650 and Duke 450 all have that extra bit of grunt and are plenty for me. I suppose 45 - 55 bhp.
However, who will break the mould with an Indian built 390KTM? That I suspect could be a lot of fun.
Guy Martin tonight on Channel4, first of a wee series, so will see what he gets up to.
All the best, Bill.