Well if it is 50% I would consider that pretty ridiculous by any rational standard. You talk as though Sheer Force isn't one of the strongest abilities in the game. It made Nidoking viable in OU lol.
Reality and theory don't usually converge in Pokémon.
Sheer Force is one of the best abilities in the game because it doesn't need any kind of planning. Sheer Force works. It's a 30% boost to your moves, as I said, from turn 1 to turn 50. It doesn't matter. Every Earth Power -or whatever- you use with Sheer Power, is 30% boosted and also negating the damaging effects of Life Orb. Calcs and all that are great, but what truly matters is what can and what can't be done easily, and how reliably it hits.
For Kingambits to receive a boost equal to Sheer Force, 3 mons in your team must be defeated. This, while said, may not sound as much if we don't pause and try to understand the authentic implications of this. I'll repeat it: for Kingambits to receive a boost equal to Sheer Force, 3 Pokémon in your team should've fainted. First, this is worse in every single sense you can imagine. Nidoking, Lando, etc., doesn't need any condition to get this bonus. It's there from the get go. Secondly, in a real battle scenario, your game plan -and win condition- changes depending on MU. What this means is the following: you aren't using Kingambit's ability in every battle. In some battles Kingambit may be an early sack; in others, you can't afford to lose 2 or 3 Pokémon, not even in the late game. What this means when we translate it's meaning again is the following: not only does Kingambits need half your team to be fainted to get a truly great bonus -not broken, just great-, but you can't afford to get this bonus in every battle. Comparatively, Intimidate, Sheer Force, Tinted Lens, Levitate, Moxie, Beast Boost or Regenerator, but I'd keep going with less broken -yet great- abilities, are there from the beginning to the end, and all of them share the same crucial trait: are non-circumstantial. They need no support and they need no gameplan involved to work. Its outcome is always the same and it's always fulfilled by just sending the mon out.
Now, in this context of true battle scenarios, do you think it's still broken that a Pokémon like Kingambit can potentially, in the best case scenario and against all odds possible, come to the battlefield as a last mon scenario, at 100% HP with no hazards nor previous chip, get a 50% maximum boost -which equals a Choice Band- and put X work as a 50 base speed mon -even if it has suckers, which relies in prediction-? I don't think it's broken at all. What are you getting in most cases? A 20 to 30% boost to put some work in one, maybe 2 mons? What are you sometimes-sometimes not (most of the times not) getting in the best of the bests of the bests cases? A Choice Band? Ok, it's great, it should be like that for those rare cases in which you play all your battle with 5 mons and Kingambit is not only your last mon sent, but also the one that can make a comeback out of 50 base speed and prediction alone.
Calcs show how powerful a Choice Band provided by 5 fainted members from a base 135 Attack plus Life Orb/Black Glasses/heck, another Choice Band is, but they don't show how often this is going to occur, and the impact of it ocurring in those cases isn't even enough to outclass mons that already exist and aren't dependant on any shenanigan to achieve the same results.