That would be close, you might not want to hold it full throttle too long. You should be fine with the 3S and 9x7 prop. Nominal voltage for the 3S is 10.8, actual charge voltage up to 12.6 volts, much higher than a 8 cell nicad/nimh which is the highest suggested amount of cells to fly with.
In KenSP's thread of his EStarter with floats, his plane looks pretty strong and powered only by a 2S with timed motor so that's another method to increase power for free.