Isn't a matter of strength & build. It a matter of speed & comfort.
The hull, deck layout and cockpit design of the First series supports racing with below deck comforts at a minimum. The Oceanis series is designed for cruisers that may occasionally race.
Hard to find a good place to mount the Bimini, Dodger & BBQ grill on a First Series.
I dont' agree with your description:
"The hull, deck layout and cockpit design of the First series supports racing with below deck comforts at a minimum. The Oceanis series is designed for cruisers that may occasionally race."
I will post the interiors of the "old" First 40.7 and the current First 40. These are not "below deck comforts at a minimum",
at least not for me:
I would describe the First as a performance cruiser that can ocasssionaly make club races and the Oceanis as a mainstream cruiser and certainly not a boat designed to race even at club racing levell.
the First is not a race boat and if you want to race the boat seriously you have to command a "special" First and not the one they sell as standard that is named CR (cruising).
Sure, the Oceanis has more space, a bigger freezer, more tankage but then the First would not need a so big diesel tank because it will sail in conditions were a Oceanis will have to motor.
Regarding comfort, the Oceanis will be more comfortable on anchor or in a Marina but the First will be more comfortable sailing with an easier sea motion, especially upwind.
Regarding the bimini, these kind of boats (performance cruisers) can use a bimini while sailing, a small one that will only protect the back of the boat, not the big one that the boats that have the traveler over the cabin can have, but then the type of sailors that sail performance cruisers will want to see the sail shape when they sail and that is impossible with a big bimini.
Anyway, if you want you can ask them to put the traveler over the cabin and then you can have a big bimini. It will not make not much sense on this type of boat and with the cruisers they are designed to, but that would not be a problem for Beneteau. About the Dodger I don't see what you mean. The First has a Dodger and not a especially small one on the option list.