I've taken the real thing in April (although voided it, as a practice run; just to see what the real thing was like. However I went in knowing I was going to void so I was very calm and able to assess the test pretty well in terms of feel). After taking both EK2 and 4 I can say that, from what I experienced on the real MCAT, EK CARS is ~35% more difficult than the real MCAT; for CARS only. I think EK exams are absolutely awesome for all science passages, it really does FEEL like the actual MCAT (so do NS, probably just ever so slightly off from EK).
Again, this is only how I felt about it though, as I didn't ever get scored on the section. All I know is that when I was taking the CARS section on the real thing I thought to myself "Wow, if I had practiced this even a little I would have a perfect score on this section." And a bunch of other thoughts along the line of "I thought this was supposed to be the hardest section? LMAO!". It did not feel like the EK CARS where you're kind of like "Hmmm, I just read about 8 paragraphs, and they're asking me some obscure question about how the author feels about such and such, when it doesn't even ****ING appear AT ALL IN THE DAMN PASSAGE!" And then they give you some annoying explanation, and you grudgingly accept it, wrap your head around it, and move on. (Again, for CARS only).
However, obviously take this advice with a grain of salt (I take it that way myself!) as each test date seems to get a different CARS. But overall I think it's reasonable to say that the EK CARS is probably longer in passage length, and definitely significantly more difficult in terms of questions and answer choices. EK tries to be very tricky with their CARS, it seems (I average 129 on TPR CARS, ranging from 8-5 wrong and 10, and 17 wrong on EK CARS for 2 and 4 respectively). In fact, I took EK4 yesterday with an average of 87% right overall and only got a 68% on CARS (36/53).
Most of EK questions for other sections are spot on. However, like every test company, there are those few questions in each section where the logic for the answer just doesn't seem right at all.