Actually late 43 and onwards they both definately had the same engine.
There is some confusion over the V in FNV.
One belief is that the 2nd stage of boost was slightly better suited to higher alts in the V which became the std after its introduction in 43.
Others claim that its a nomenclature error and infact the same engine was post fixed either FN or FN(V)
The performance difference between La5FN and La7 is essentially from weight, drag and prop differences.