Multiple people have already made it clear that Yanma and Carvanha aren't broken without Speed Boost, including myself in our IRC discussions, so I feel no need to re-cover that information in this post. Rather, I'd like to address the issue of complexity that tier leader Elevator Music believes is the main issue.
I'd like to remind you all that Little Cup already has the most complex ruleset and banlist of any tier officially supported by Smogon by far. The laundry list of differences and quirks from the level 100 tiers is highly confusing to new players on its own. I myself was originally off-put by the strange level 5 EVs, and found them difficult to learn. Even today, I have not memorized how many EVs it takes to max out a stat that ends in a certain number, and can't tell you the max stat you can get just by looking at the base stat. Also, there is how some Pokemon have the same stat as others even though their base stats are different. A Base 70 Speed Pokemon is not always faster than a base 65 Pokemon, and if both max out their Speed EVs and have a positive nature, then they speed tie, even though one has a higher base stat.
Then you have all of the illegalities and tier-specific bans. It's easy to explain why Scyther and Berry Juice are banned to a newcomer, but it is far harder to grasp why not everything can have its DW ability even though it is released, why some genderless Pokemon can learn level-up moves and some cannot, how the Pomeg glitch from gen 3 affects those Pokemon, up to how the same Pokemon (Porygon) is legal with some moves and illegal with others just because of its ability. We have banned moves and items, which no other tier does, and have unbanned some stuff banned in the upper tiers, such as Sand Veil and Snow Cloak. Little Cup is certainly one of the most confusing tiers to learn.
What does banning Speed Boost do to increase the complexity of the tier? Is that the straw that breaks the camel's back? I don't believe so. I think that banning an ability is something far more intuitive and easy to understand than why Porygon can't learn all of its moves at level 5 when it has Download. If you know what Speed Boost does, then you would think "Hmm, an ability that boosts Speed passively is broken in a tier where Speed is one of the most important stats. That seems pretty easy to understand." I realize the bulk of this argument rests on "We have done worse, so this is fine," my reason for doing this is to point out that Little Cup is not a simple tier by its very design. This is a metagame that is as far from the other tiers as humanly possible, far away from most of the strategies and guidelines for playing that work well everywhere else.
About Pure Power, we do have non-broken examples of a Pokemon having Pure Power, and that is Azurill. Even with doubled Attack, it is still very mediocre and arguably outclassed by Eevee and Aipom due to their higher stats. I believe it was blarajan who commented on this saying that it was just coincidence that everything that got Speed Boost was good and that not everything that got Pure/Huge Power was good. This, to me, is a moot point. As a tier, we should be focused on what "is", rather than what "could". Nintendo could release an event Magikarp with Speed Boost in a month if it wanted to, but I am not concerned about that. At this moment in time, Speed Boost Magikarp does not exist, and even if it "could" in the future, right now it is a non-issue. Same with Speed Boost Torchic, in that it is does not exist in Little Cup, and is only paid attention to because of the likelihood that it will be released for LC use at some point. That's fine, but as of right now, Torchic does not have Speed Boost, and is therefore irrelevant. In this metagame, Yanma and Carvanha are the only Pokemon with Speed Boost, and are therefore the only Pokemon that we should be taking into consideration when we discuss Speed Boost.
Put simply, Little Cup is already a very complex tier. I don't think we can make it more complex by banning an ability, and as such, I feel we should take any step that has a chance of expanding the metagame.