All ASIC Miner are specified with optimum Gh/s +/- 10%. In your case if the optimum amount is 270-275Gh/s, you calculate -10% and you are still within the specification.

It all depends on the difficulty.  It's rather easy to project what the next difficulty jump will be, however it is much less accurate to predict where the difficulty will be in a few months, a year, etc.

Edit:  if you want to mine for profit, I suggest a Scrypt GPU mining rig for starters.  Cheap, you can resell the GPUs, and they are profitable.  ASICs are much more unpredictable and have little-to-no resale value after they become obsolete.