Hi Snufkin - as you've speculated, if you're going to start riding with 30-40Kg of luggage onboard, then you're unlikely to be fully exploiting the benefits of a smaller/lighter 'trail' bike anyway, so yes, the difference is not going to be quite so marked - although the same would be said for a larger bike carrying the same sort of luggage load too of course, so the smaller bike is still that much lighter to pick up of you drop it for example...

I think as you've ascertained, the requirements for a longer-distance travel bike are not the same as they would be for a single day-out trail-riding in tougher terrain... much more of a priority is going to be comfort and reliability, luggage carrying capacity and longer maintenance intervals etc. - all of which changes your priorities from the outright 'performance' of your bike to the 'overall' performance if you see what I mean?

If you like to take the smaller roads - ie. 100kmh and below type riding, then a smaller capacity bike still has a lot to recommend it (as long as it has a comfortable seat and decent luggage carrying capability), but if you want to hammer down a few hundred kms of Autoroute before (or after) the main part of your trip, then yes, a larger capacity bike is going to make easier work of that kind of riding - which is why a lot of people still chose a 650cc class single-cylinder bike (KLR/XR/DR/GS/KTM690) as it does have the edge over the smaller capacity trail bikes on the open road, but is typically still not quite as large or bulky as a 750+cc twin 'Adventure' bike once you get it on the dirt.

Personally, if you do plan to ride a lot of highway miles, and don't really intend to ride 'hard-core' off road on this particular bike, then for multi-surface adventure riding with luggage, I'd recommend you look at a Honda CB500X with a Rally-Raid wheel and suspension kit fitted - it neatly bridges the gap between a 650cc single off-road, while offering the on-road refinement of a larger twin.

Hope that helps...

Jenny x