Venusaur, good defender?

That's the perfect defensive move set for Venusaur, so I'd say hell yah, definitely power him up!
As to why it makes such a good defender, I'm not sure exactly why but I'm guessing because it's a decent counter to a lot of Pokemon, especially when it has a high HP and a good defensive move set.

I fought a 2300ish one the other day and it was surprisingly good and it had petal not sludge.

Go for it, break the mold. Too many of the same type in gyms. I'm sure a lot of people use multiple Vapes to attack still too. Would be a good counter with its grass quick move which is venusaurs main selling point.

It's the extra damage they do against water types e.g. the common Vaperon attackers that make them special.