Jesu, is this a perfect future (not a future perfect) :
« Et facile intelligo me, quatenus rationem habeo totius cujuſdam, perfectionem futurum fuiſſe quàm nunc ſum, ſi talis a Deo factus eſſem. »
hi, the (vocab-heavy) answer is that FVTVRVM FVISSE in this sentence is the periphrasis you use for the verb ESSE in the apodosis of a contrary-to-fact condition in indirect discourse. see a&g s589(b)(2): http://www.archive.org/stream/allenandgreenou02greegoog#page/n401/mode/1up
to explain that a bit, in direct discourse this person would be saying basically:
protasis (if-clause): SI TALIS A DEO FACTVS ESSEM,
apodosis (then-clause): PERFECTIOR ESSEM…
i.e. the imperfect subjunctive would be used in the apodosis for a contrary-to-fact condition in direct discourse referring to the present.
then when you put it into indirect discourse (with the verb INTELLEGO introducing indirect discourse):
INTELLEGO,
if-clause stays the same: SI TALIS A DEO FACTVS ESSEM,
in the then-clause of a contrary-to-fact condition in indirect discourse, the imperfect subjunctive ESSEM turns into a -VRVS FVISSE construction: ME PERFECTIOREM FVTVRVM FVISSE… (for this see a&g s589(b)(2) linked above)
(presumably in your original it said PERFECTIOREM, ie the comparative of PERFECTVS in the accusative agreeing with ME, and not PERFECTIONEM, unless i’m wrong… 
)
cheers, chad
Thanks, mate, sorry about the misspell.