If we're going to compare Amoonguss to M-Venu, let's not just leave out the negative aspects about Amoonguss. Venusaur's main benefit over Amoonguss is its far greater offensive presence, which allow it to still be more threatening to teams as opposed to Amoonguss who kinda sits there once it has thrown out a Spore since its damage output uninvested is pretty damn poor, especially if you're running Clear Smog over Sludge Bomb. Granted, Amoonguss is great at pivoting around during the time that it has used Spore, but its passiveness still must be stated.
I feel that Venusaur should definitely drop, but Amoonguss' main competition as a splashable bulky grass type is Tangrowth and they are about on the same level of viability, so I see no reason why Amoonguss should rise a rank above Tangrowth, unless you are willing to raise Tangrowth to A- as well along with Amoonguss (which I certainly think is a good idea, but that's a nom for another day) So overall, drop M-Venusaur to B+, and keep Amoonguss in B+ as well.