GMAT Club CAT questions are one of the toughest and most detailed ones..
The quant part of the CAT have the best questions that you will find anywhere. And it's not just the questions themselves, the explanations provided also help you uncover many shortcuts and help strengthen your core concepts. And also, you can take the quant section or verbal section one at a time, which gives great flexibility when you are hard pressed for time.
I have used the GMAT Club Tests for a while now in the past and I personally believe that the level of questions is very advanced. Once you start solving these questions, you might realize the difficulty of 700+ Level questions and you will be prepared for them.