Hmm. Well I just added a porteur rack to my soma Buena vista (650b). It has mid fork mounting braze ons.

I have ridden with 10-12# loads a few times now. Each positioned slightly differently. Other than being a little harder to get lined up with my kickstand, and a bit more wobbly at take off (when I first get on the bike), it is fine.

I have slight to moderate hills all around. And of course it is a bit harder than unloaded. But not hugely different.

The porteur handles better than my old basket!

I also rode a Dutch style (700c) bike with a front rack and basket. It worked fine as long as the heavy part of the bag was closer to the fork and rack. (Top heavy loads were hardest).

I think being able to mount lower and on the fork is most key for better handling.