Nice agressive game. I have never played this variant, but it seems as if Tyrell may be to easy of a mark by utilizing Sunset Sea and Golden Sound as a single region. Does it seem that Tyrell is a fast early mark?
I was the Tyrell player in this specific game (a few times prior as well) and have never had any issue as far as being an "easy mark".
Moving out into West Summer Sea and keeping ships for support in the Redwyne Straights generally acts as a good enough security blanket to keep people honest when dealing with this situation. Also if you get an overly aggressive Lannister Navy a lot of times they will end up leaving themselves exposed for a Tyrell (or anyone else) attack on Lannisport with little to no sea support to help them, this is what I was hoping would happen in the last game until Joe swooped in for the win.
Having a general understanding of the tendencies and playing styles of the people you are playing with certainly helps as well.