I don't think Martins has the acceleration or agility to play centre....I don't think Fakir does any more either. Fakir's strength is hitting the line hard from depth, and he has great timing when doing so meaning he nearly always hits the ball at top speed (or if he doesn't, it measn his true top speed is scary!). Ideas of putting him anywhere else other than the back row would be folly. If it ain't broken....
Martins is just too big! He is an out and out prop and I'm sure the French development staff recognise this. Again, tinkering with him positionally would be a mistake.
The French national side has made such stellar progress I am sure that within 12 months, when they will have mostly had a year's full time professionalism under their belts, they should be a level match for England. They're already superior to the SL packed HNs sides.