...By the way I believe the ones requested by SW-Motech on the plate holder would not provide much strength.....
I had the exact same concern, but then there was not a chance that I would dismantle half the bike to install the GIVI system.
On the other hand SW-Motech is a well known company for quality and trusted systems. I have friends that have the same system on their KTMs which is mounted in a similar manner and they never had a problem even doing off-road.
So, I took my chances and indeed the system is very rigid.
After all, this "bridge" does not carry vertical load. It is subjected mainly to horizontal force in between the two racks.
Consider that, due to limited space where I park my bike, I have to pull it (on tiles of course), and the only part that gives a good grip for this task is the side rack. So, I pull the whole bike for some half a meter or so, only by applying pull force on the side rack. It fully withstands the stress.
All in all, there should be no consideration for the strength of the system.