I've been so busy the last couple of months, but I'm still doing this guys, and I'm happy enthusiasm has grown while I was away.
I agree that the transport would be AV 11-11-10, fast, skimmer, open topped. I think there would be two variants of this, a transport that holds 10 models and a gunship (im thinking of a combination of a raider/ravager and a vyper).
I don't know about letting them get the best of both worlds for the vehicle though. If this is to be used then opponents can't feel like its over powered.
I think walkers may be used occasionally, for scouting, but I agree no monstrous creatures/ or wraith units.
It will be extremely fast, but not as fast as the DE. Eldar and Dark Eldar already are capable of running rings around opponents, but I'm thinking corsairs would sacrifice some of the DE speed for a little more protection on their vehicles. See above vehicle idea. As for shooty vs. close combat, I'd rather have a codex that can be used to kit units the way an individual wants.
I just can't see a Farseer just getting up and leaving a Craftworld. The only psykers would be at Warlock level, once walked the path of the Seer, then left. Maybe invent some other warlock powers. As warlocks don't need to take psychic tests, maybe create a system where every turn you roll a D6 and on a roll of a 4/5+ they have to take a perils of the warp test. That way they are still just as likely to survive a perils, but it would happen more frequently (as they aren't as trained).
For the regular squad (say 10 men) 2 of them could replace the 'pirate rifle' created earlier with a flamer or melta at X points, and 1 model could replace his pirate rifle with a heavy weapon.
The assault sqauds of pirates would be armed with cc weapons + pistol and 2 models could replace them with a flamer or melta at X points (like storm guardians). They cannot take a heavy weapon, but an additional 1 - 2 models could replace the cc weapon and pistol with a special close combat weapon.
Alternatively, following the idea that pirates don't really get to pick and choose what they're armed with, it could be that the regular squad of pirates has the choice of either a pirate rifle or a pistol and cc weapon. 4 models in the unit would then be allowed to upgrade to either a flamer, melta, special close combat weapon or heavy weapon. While this would be fluffy, I think it game terms it would be to much and the unit wouldn't be very effective at doing anything.