I have a 123" diagonal Firehawk with my S2 and it's plenty bright. You will be fine with the firehawk, I have no experience with other brands of high gain screens so I wont comment if you can get the same screen cheaper somewhere else.
I saw an S2 on a 133" diagonal firehawk. In its dimmest mode all four of us at the demo thought it needed to be brighter when protraying dim, low contrast scenes. In medium and bright scenes it was fine. In its brightest mode, it was bright enough for any movie scene (eg Dark City) and yielded a beautiful picture.
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