I think the feeling is that TPR tests are pretty similar to the AAMC ones. Some people find them easier, some harder. There is a general concensus that the TPR verbal is a bit more difficult, but not by much. Personally, I don't think you should add any points to your TPR tests. With your scores, it seems you just need to brush up on the material. I think taking the tests without covering the material first is a bit of a waste of time, besides telling you that you are woefully under prepared.
My first TPR diag was a 20 (P6 V7 B7), and my second, three weeks later, was a 22 (P7 V7 B8). I did a third test about a month before the MCAT and got a 29 (P8 V11 B10). That's all the tests I was actually able to fit in because I was trying to make sure I covered all the material to a sufficient depth (which I finished the day before). I'm sure I would have scored better than 29 after I covered all the material.
My suggestion is don't worry about how predictive the scores from Kaplan, TPR, EK, etc. are. Use them as rough guides. If you get 20 like I did, rest assured you won't be getting much, if any, better than that on test day. Get through all the material first. Then do practice passages and all the tests you can get your hands on (Kaplan, TPR, EX, AAMC). They won't all be exactly like the MCAT, but they'll help give you the skills needed to do well.