Hi again,
On Sunday I took time to test the bike with 18” wheels – a 10km ride across the old town. I must warn you readers, none of my findings will come as a big surprise!
I didn’t adjust the height of the saddle so it’s higher now – an unanticipated, but welcome side-effect.
The bike definitely feels more stable, which is good, while remaining very agile in town. I must admit that as a typical Frenchman, culturally (and pathetically) allergic to petty regulations, I occasionally ride on side-walks so agility is important for the safeguard of my fellow pedestrians. The beast also feels quicker, very logically, but that’s a feature that I fully enjoyed only this morning as I was riding to work on long straight lines. There are hardly any slopes in my flat region so that’s definitely the right choice. lastly, these wheels just… look better 
On the downside, the thinner tyres do make the ride rougher. That’s a fact and it was to be expected. At one point I rode across the oldest streets of Lille, which are mostly made of disjointed cobblestones, and I felt like my skeleton was going to fall apart. And the palms of my hand and wrists were in real pain – so, never again will I ride these streets with the Strida! Maybe you know about that bicycle race “Paris-Roubaix” with its famous paths of cobblestones… I wonder how participants can survive such a treatment. Anyway, that is an extreme situation and I guess it would have been painful with the 16” wheels too, just maybe more acceptable. Having said that, on the normal streets and all bicycle lanes I’m likely to use on a daily basis, it may be also a bit rougher, but not to the point it becomes unpleasant. The bike does “vibrate” a bit more, and I noticed the bell (which was already noisy) was “signing” its unwanted tune all along… so I just got rid of it. Problem solved !
Lastly, fitting those bigger wheels has few measurable consequences when the bike is folded. Admittedly, I have to lift it higher when carrying it through stairways and escalators… but that’s because I’m shortish, at 1.74cm At floor level the bigger size of the wheels causes no problem, not even in a packed metro, as I could experience this morning. It just makes things a wee bit more complicated on escalators: the 16” wheels could fit perpendicularly to the stairs, making it easy to start folding/unfolding the handlebars and pedals while going up or down. The 18” wheels only fit parallel to the stairs, making for a slightly more cumbersome position of the bike, that’s no big deal, just a question of habit.
As a conclusion, I’m happy with the new 18” wheels, definitely no regrets. And the travelling time between my office and the metro station has decreased by a significant margin