I understand where you're coming from, but I really wouldn't think of it as "give me a bank of 600-700 questions and I'll do only those." Even if such a thing existed, I would tell you not to use it. :) Basically, that kind of grouping doesn't serve you well because there are some areas where you're totally fine already with 6-7 level Qs and others where you might struggle even with 5-6 level Qs. Your strengths and weaknesses will always have a range across different kinds of problems and topics.
The real isse: can you can do the one question that's sitting in front of you right now, both accurately and efficiently? It ultimately doesn't matter what the question's level is - just, can you do this one right now?
Now, yes, if you're going to do question sets from OG and you discover, oh, when I do questions numbered lower than (for example) 30, I answer them all correctly in 1m each - okay, those are too easy for you, so move up the food chain to higher-numbered problems. But there might be others that, right now, you're answering just fine in 1.5m or 2m... and yet you could actually learn how to shave 10 or 15 or 30 seconds off of those problems, and you could actually use those to learn to spot certain traps or figure out how to make educated guesses, all of which will help you on harder problems of the same type. So you don't want to skip / neglect those just because you can already do them.
The MOST important thing is analyzing the problems (to death!)
after you've finished doing them so that you're extracting maximum possible learning from each one.
Read the "how to learn" section of this article:
http://www.manhattangmat.com/blog/index ... an-part-2/And follow the links to the other articles mentioned in that section.
So, very generally, don't do the very lowest-numbered OG problems or the very highest-numbered ones, but really the vast bulk in the middle is fair game for you right now. You don't want to do *only* the ones that are hard for you right now, because there's still a lot to learn on ones that you can already get right.
You mention some timing issues and that's all tied up in this as well. Often, we learn more efficient ways of doing something, or we learn how to recognize what to do, by examining / thinking about problems that we DID get right already. Again, see the stuff up above about how to learn.
Finally, your last line mentions that you've got 3 weeks till exam day. How far are you from your goal score? Just keep an eye on that - things really start to come together at the end of the course and after it's over, because that's when you can start to focus more on the big picture and overall strategy vs. learning very specific rules, techniques, etc. That's also when the timing starts to come together better. You may discover that you would benefit from an extra week or two if you feel like your progress is really starting to ramp up over the next couple of weeks.