Not tried out Specs Yanmega much so I will have to have a look at that.
The trouble with Sharpedo though is that it cannot OHKO a lot of offensive pokes at full health. For example, Jolly Sharpedo can't even OHKO Meinshao, one of the most frail sweepers at full health (Sharpedo@Life Orb (252 EVs, Neutral Nature) Waterfall vs Meinshao (0/0 EVs, Neutral Nature): 90 ~ 106.2% (244 ~ 288 HP)). On the flipside, almost any neutral hit from a sweeper can OHKO it.
So Sharpedo needs more than just Yanmega to form a core. Sharpedo really needs Stealth Rock and Spikes on the field in order to function at its best, so Roserade and Bronzong would make great team-mates. Bronzong would be the preferred Stealth Rocker since you desperately need a Physical Wall if you're using the aforementioned pokes, and he also provides a much needed Flying resist which is completely necessary if you're also using Hitmontop to keep away the rocks Yanmega hates. That's not a core, that's practically a whole team.
If I haven't seen further, it's because giants were standing on my shoulders...