I find the Grand Espace handling amazingly unaffected by loads in the back.
I used to have a GMC El Camino pickup with adjustable airbags. Being American it really needed it to retain any handling with a live rear axle.
I'd be more concerned about what that top box is doing for your fuel consumption. The Espace roof bars are there to look cool, not carry things
I sometimes carry bikes on my roof and it knocks about 6mpg off on the motorway.
I can believe that the El Camino didn't do well in the twisty section of the "track"...
Sadly the roof box is a bit of a must have... After loading the bikes up, there's not much space in the car anymore.
See attached pictures - space in the car, and what gear we need to bring. In addition to that we need our normal "travel gear" - clean clothes, toiletries, gadgets, loading ramp, etc, and we also need to transport 2 exhausts per bike (one standard and one louder) in case we're not allowed to use the louder can. It all mounts up quite a bit.
... but I guess you're right - it'll add to the MPGs... I usually get 32 MPG doing my normal teeth-grinding-white-knuckled-commute, so a steady motorway pace shouldn't be too much worse than that, even with the roof box and the load... but we'll see...
Thanks!