I think it really it has to be two different discussions:

Less than 8 players:
(Assuming the vassals are below Baratheon on the Kings Court track) The iron throne is massively more useful. The vassal system makes NPC houses much more potent, and with Baratheon a) being a centrally located house, and b) being able to dictate the pace of early Westeros cards, they are in a prime spot to take advantage of the vassal system--with the only house that might have a better advantage being Lannister.

8 players:
Baratheon is clearly in the most dire spot of all players. Every neighbor has better cards, or more deadly cards, or a better starting army, or all three. Combine this with the fact that Baratheon *still* has to deal with the neutral force on King's Landing (with no way to do so on the first turn, nor on the second without committing all land forces to do it), before getting their second stronghold, and they will have an exceedingly difficult hill to climb. Diplomacy will have to be central to any strategy--particularly because Targaryen will likely have no interest in an alliance with Baratheon.  Loans will also likely be a major factor for Baratheon because they also get first dibs, but, again, that's a major sacrifice for not being able to take King's Landing.

Tyrell:
This should open a number of new doors for Tyrell to not only get started, but also defend themselves.

Realistically, their best move is to CP* in Highgarden, because a second/third boat will automatically equalize/neutralize the biggest threat from Martell. This leaves the only difficult decision to be what to do in the Marches. Stay and CP for tokens or take Oldtown for future mustering? Given how important tokens are in the expansion, the safer option is likely to CP, but an argument could definitely be made to take Oldtown to beging building up for the move against the other houses.

Begin Rant:
Tyrell, in my opinion, is now in the de facto best position of all houses: good geographic position, good cards, and good starting tracks. They will become the new Starks, I think, able to turtle better than any other house.

Baratheon is really hurt by this... I would really have preferred the addition of another one star position to Kings Court *or* getting rid of the King's Landing Neutral force and replaced it with a footmen for Baratheon or reduced it to 3.

It would be one thing if Baratheon had decent cards, but they really don't, and now Baratheon has to commit an offensive move to get decent mustering. I'd welcome an analysis arguing otherwise, because I'd like to be wrong on this.

There's also the issue of Greyjoy now being to march the boat out of port with the new sea order move and still being able to threaten both Seagate,Riverrun, and the Golden Sound.