Hey now, I'm Ishukone 4 life. But my thinking tends to be that the pirate factions (while they certainly have populations in the many billions) have power in a different form, mostly, than megacorps or other 'subfactions'. It is money, cornered (mostly illegal) markets, and webs of influence. It's a piece of every sports bet made off the corporate books and every little baggie an up-and-coming junior executive buys to get him through his next product launch, every special something that isn't available through normal channels or with corporate scrip. Maybe you are right, or maybe it's more that they might rank in the midst of the Megacorps, or the other analogous sub-factions. I think if they do, though, it's near the top and I honestly think if there was a way to make a head-to-head comparison they would come out ahead. I think someone once described the Guristas as the ninth Megacorp, which is probably close to accurate.
And I think if they were less powerful than those fractions of the empires then they wouldn't still be around. It might vary a bit by pirate faction, too. I'd say probably the Serps are the weakest, or maybe just the closest to being basically a traditional Corp. Blood Raiders, it's kinda hard to tell, but I'd put them and the Sansha up there. Angels are probably the most powerful, though quite possibly the most diffuse. But rather than weakening them I think that jsut makes them that much more pervasive - they've even tried to move in on Gurista turf before (that thing with, who was it, NOH?)
EOH and Mordu's I would definitely class well below megacorp level.