Don't know if you're still looking for answers, but I've heard that reading the Wall Street Journal improves reading comprehension for scientific texts. Scientific American might also be helpful too. In terms of test prep material, I liked the DAT Bootcamp RC material.
I think finding and practicing a strategy that works for you is equally important. I'd guess that even many native English speakers would have problems reading an entire passage thoroughly and answering all the questions within the time limit. Since there is not enough time to read thoroughly, you need to find a way to navigate through the text quickly to find your answers. In other words, don't read to understand the material; read just enough to know where to look for the answers. Once you know where to look for the answers, you can read those parts a little more closely if you need to.
I've always been pretty successful with RC/Verbal sections on standardized tests. What's worked well for me is to skim the entire passage first, then answer questions. Skimming first provides valuable context, which helps me to interpret things correctly, and also helps me understand how the information is organized/laid out. This, in turn, helps me work more efficiently and make fewer mistakes.
So, what I like to do:
1. Skim the entire passage, noting HOW the passage is structured/organized. *Important: Don't get bogged down in the details while you are skimming.* You are not reading for the sake of understanding every single word or sentence, or even all the ideas in the passage. I usually skim by reading just the first couple sentences of each paragraph.
2. Read question 1.
3. Locate the part of the text that contains the answer to your question. Since you already know how the passage is structured/organized, you should be able to locate the relevant section quickly. You can then focus on reading and understanding that part of the text more carefully.
4. Repeat 2-3 with remaining questions until all questions are answered.