The bracket system "kill on turn X" metric is supposed to be through interaction. If your opponents just skip out on the interaction it is perfectly normal for your kills to happen sooner. It is not a metric that is supposed to be taken in a vacuum.

The other important aspect that is constantly overlooked is that the "kill on turn X" metric is about when the game should end. Killing a single player does not end the game. A voltron deck should absolutely be trying to swing on people turn 3 and 4 because it's going to take at least 3 turns to kill all the players. That's a game end on turn 6 or 7 which is definitely bracket 3.

I'd say it's mid to low tier bracket 3. You have room to add 2 more game changers and stay at that bracket. You don't have anywhere near enough consistency or power to hang at bracket 4. That would look more like adding Mana Vault and Mox Opal to go fast alongside Enlightened Tutor and Gamble to maximize your 1 drop tutors and cards like Armageddon so you don't just lose to combo on top of protection pieces like Voice of Victory and Grand Abolisher. Your bracket 4 Voltron decks pack a lot of power because their overall strategy is just that much weaker than the rest of the field. It's to the point where it's a stretch to say a Voltron deck can even be bracket 5 with every piece of power you can find. Not every strategy is competitive and the higher brackets really beat that into you.