Similar base stat total != statistically similar. For in-game purposes, attack stats are what matter most, and Feraligatr severely outclasses Tentacruel. Feraligatr has a 105 attack and 79 special attack. Tentacruel's best attack stat, special attack, just barely beats out Feraligatr's worst attack stat, at 80 base, and Tentacruel's attack is even worse. Feraligatr has a better stat total than Tentacruel, but more importantly, he has stats in categories where it matters.Tentacruel is really pretty much equal with Feraligatr stat wise, and it has dual STABS and better resistances, so I can't see them not being in the same tier.
Plus, you don't even get to make the Feraligatr/Tentacruel comparison until level 30. Until then it's Totodile/Tentacool, which Totodile wins, and Croconaw/Tentacool, which Croconaw wins even harder. You don't spend the entire game in the 30+ level range. In fact, what comes before that point is what tends to matter most.
Tentacool is quite mediocre. I think the main reason it's so high is its utility factor (it's one of the best HM slaves in the game and it's available from very early on), but I'm thinking that perhaps it should be lowered. It seems to be about as good as Chinchou (water hybrid, tank, slow experience group), though possibly worse for the fact that Chinchou naturally learns a decent special non-water STAB while Tentacool needs a TM to learn Sludge Bomb.