While reading highbrow magazine articles in English can help improve your overall reading skills, in order to REALLY conquer GMAT RC, you need a solid strategy for (1) interpreting the passage AS YOU READ, (2) writing down your own answer BEFORE looking at the answer choices, and (3) using decisive, thoughtful analysis of all choices during process of elimination to narrow it down to the inevitable correct answer.

You need familiarity with how the GMAT presents information in its passages, what the major question types are, how you can simplify question stems so that they seem simple and understandable. Basically, you've got to be the Sherlock Holmes of GMAT passages.

So I actually think you should spend a LOT of time honing in on your GMAT passage strategy FIRST, and only a minimal amount of time reading non-GMAT material. It's not your READING skills that need to be improved, it's your ANALYSIS skills. Most people don't really work on the latter, which is why their RC ability stagnates.