GMAT questions on the exam a lot harder than many practice questions?

1. More and more high scores: this is not true, or it is extremely unlikely to be true. At least where it matters. The average GMAT (total) score has not changed much in the last 10 years or so (only quant percentiles have changed significantly).

2. Old material is too easy: how old? OG questions are still very representative of what the GMAT tests. You should be fine using them, as long as you keep in mind that the OGs are not meant to provide you with only "700-800 level" questions. The GMAT itself does not have a particular difficulty level (it's an adaptive test). So yes, if you are very good, most OG questions will be easier than the ones you'll see on the exam.

3. Doing "600-700 level" questions correctly: it's very hard to put questions into such categories, and the GMAT itself does not use them. Use the GMATPrep practice tests to get a better idea of where you are. Do your practice sets/drills under timed conditions.

4. False sense of security: (from the earlier point) don't use practice questions to determine your current ability level. Use the GMATPreps. Even the GMATPreps won't account for exam day pressure though.