I am a native English speaker, and I do extremely well on the verbal portions of the GMAT, particularly SC. I started going through the SC 1000, and was stunned by the number of errors within the answer choices - many of them appear to be poorly transcribed from other sources - and also the errors within the answer keys. For those of you trying to bring up your SC score, I strongly advise you to go through a copy of Manhattan GMAT
's SC guide with a fine tooth comb - do flashcards, take notes, etc - and then reload the official GMATprep software so that you can take the verbal section over and over again. Yes, you will see many of the same questions at first, but as you become more proficient, you will see other, more difficult questions coming at you from the bin. I've used this wonderful website extensively to help me with my quant, and I'm grateful. This piece of advice re: SC is part of my thank you to those of you struggling with your verbal score. Please heed my words!
...there ain't no such thing as a free lunch...