Hi there

First of all, something you should keep in mind and that I personally find awesome is that each Insanity workout video has a warm up. In case you didn't notice, "Strength" in no exception. Therefore, there is no point in warming up twice.
I completely relate to the feeling you have. You don't feel the workout is hard "enough" or you don't feel exausted the way you believe you should be. However, you have been doing Insanity for a while now, which means your body not only has adapted to the effort, but you can do more without feeling as tired. If you beat your body up more and more everyday you will probably give in. This said, give it your best and don't worry too much. As long as you do the work you will still get the results.
Hope this helped.