Hi BP,
By the very way it's designed, the GMAT has a limit to how many truly crazy/hard questions it can possibly throw at you. Those same crazy/hard questions tend to come from certain categories (multi-shape geometry, permutations, combinations, probability), but they can also be found in certain types of algebra or arithmetic. Sometimes the questions test rarer principles; sometimes they test you on an idea that you know, but in a way that you're not used to thinking about it.
On your best day, you'll only see about a half dozen questions per section that will truly make you think "wow, this is freaking hard." For that reason, many Test Prep companies are put in the position to "design" their own versions of "hard questions" because the amount of available source material is limited. On one hand, those designed hard questions are usually based on rules/concepts that the GMAT tests, so aspects of those questions ARE relevant. On the other hand, sometimes the questions don't look anything like how an actual GMAT question would look, so the overall relevance is sketchy.
Based on your goals, you don't need to worry about any of those crazy questions. Your missing points can be found on mid-level questions AND when you eliminate any existing silly mistakes from your process.
GMAT assassins aren't born, they're made,
Rich
