I don't see how one could find more pronounced and greater upheaval at any rate than what happened to the Romans in our time.
You have two comparatives and then you have τῆς συμβάσης for your genitive of comparison. What more do you need?
Neither Perseus nor the French has a partitive. I translated the gh as "at any rate". Perhaps there is something better.
I need an explanation for the τῶν which I'm still not convinced forms a genitive of time
with καθ' ἡμᾶς, as the latter already forms a temporal sense of "in our time/during our times".
and therefore being unable to find in our day a more rapid or more signal change than that which has happened to Rome, I reserved my disquistion on its constitution for this place.
You don't see here a partitive because of the translator's style, phrasing and word order.
The Greek word order would suggest μεταβολὴν is taken with τῶν καθ᾽ ἡμᾶς to mean
"amongst those changes in our time, he can't find a more rapid or more signal change than that which has happened to Rome".
He's canvassing the extant historical documents of known events, and hey, he could only find
one amongst them which is suited for his upcoming treatise.
Maybe I'm being overly protective of the partitive genitive, but that's just how I understand