Taking a turn to set up Sticky Web and using Galvantula is taking a turn and slot away from setting up Stealth Rock instead, which is a much bigger game changer. I also find Sticky Web to be highly outclassed as a speed modifier by Tailwind. Tailwind is limited but it is guaranteed to affect your whole team and isn't dependent on the enemy's team. And there are TONS of good Tailwind users like Talonflame, Staraptor and Latias just to name a few, and they all have many more uses outside Tailwind. Tailwind is limited, but it's certainly easier to use than either Trick Room or Sticky Web, and your opponent can't end it prematurely. This can give you just enough time to cause severe devastation with someone like Landorus-I, Kyurem-B or even Darmanitan! Even Sticky Web is sooo niche that it is more often than not not worth the hassle and I would put both Galvantuala, and even the move itself firmly in C tier. Galvantula is not as good as pokes like Mega Aerodactyl, Celebi, Krookodile or Tangrowth.
Actually looking at the C tier again, Porygon 2 is not justified there, as it is certainly better qualified than Blissey C+, being far better at mixed tanking AND special attacking, with Trace being a phenomenal ability allowing it to stall out balance, and outstall stall! Depending on what you Trace. But in a meta where people are nominating Clefable for S giving Porygon 2 the ability to Trace Unaware or Magic Guard? Just say gg right there man. He can potentially tank as well, if not better, than Suicune and Gliscor who are B+. So I say Porygon 2 for B/B+. Status can be an issue unlike for Gliscor and Suicune, which is why B is probably more appropriate, but with abilities to trace like Magic Guard, Poison Heal, Natural Cure and Magic Bounce, this isn't necessarily too much of an issue. Not to mention a simple cleric with Wish is all the support a Pory2 really needs.