Is it worth studying Kaplan 800

This is a bit difficult to answer, as these are different products and we don't know your preparation so far. The Kaplan 800 book is basically a collection of practice questions in the 650 to 700 range, while the Powerscore books are guidebooks. It depends on what you have done so far in terms of study and your current skill level. If you have already completed a math guidebook and are reasonably strong in that area, then maybe more time can be spent on verbal. This argument becomes stronger if you haven't even started on verbal yet.

I can continue to speculate, but we need more information about what's going on with your test preparation. How strong are you on math and verbal? Have you previously used guidebooks for verbal or math? Have you used the OG? How much time do you have left?