I absolutely agree with you that the AMC problems are an excellent set of problems, and very much relevant for the high scorers on the GMAT. I recently wrote about it in this post:improving-q-from-a-50-to-a-145212.html
I actually went through all the AMC questions and curated and assembled only those questions that I believe are relevant for GMAT test takers. Here is the link to all such problems on my site:http://www.gmatquantum.com/amc-questions
I do want to emphasize that many of these problems are not meant to be solved in 2 minutes, actually many can be, after all the test allocates three minutes per problem! But they are tricky and for GMAT purpose I would recommend just struggling with them and forcing your brain to think in as many different ways as possible.
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