I did the OG12 and the Quantitative Review 2nd Edition. I logged problems, repeated hard ones and ultimately I feel I've mastered most of the material.
Then I did the delta problems for OG13. They were very hard... and yes it ruined my confidence for a day or two. The DS was OK, a bit hard, but the PS was particularly hard. I also don't believe the way they arrange the problems is correct AT ALL because the last two OG13 problems that are new are very easy (200s or something), whereas the 4 before that are as hard as I've ever seen. (180s-190s).
What the previous poster said is absolutely correct. If you lose confidence right before your test it can be disastrous, and that's probably why they say don't study anything new immediately before. I think the only thing you should be concerned about is confidence...not that you have all this "shadow" material you don't understand.
My opinion is,
If the GMAT is a well designed test, then ultimately it needs to gauge you as an individual properly. That means both revealing your weaknesses AND revealing your strengths. If the test gives a 750 test taker a 650 by happening to introduce all this non-representative stuff (kind of like us doing the OG13 problems, which are a mere fraction of the scope) then the test is flawed period. Likewise, if a 650 test taker comes out with a 750 then the test if flawed too -- the job of the test is to find your ability, not screw you. So the way I see it is the testing mechanism works in favor of people like us. People that have covered/mastered the most representative material available for preparation.
I think it also helps to realize that at this moment there is enough material out there to take more than a year to cover. There are half a dozen test prep companies with their own problems. Even forums have their own problems (like this one) and their own guides (like this one). Some people even study off LSAT guides for Critical Reasoning.. you're never really done if that's the way you define "done". So that's why I just define "done" as as much of the official material you can reasonably do in a 3-4 month time period.