I can tell you that I regularly use the 7d as a B cam to the 5d3 (both RAW).
Go to 0:23 on this video. You'll see three different angles of the interviewee one after another. Two of them are 5d, one of them is 7d. I bet you can't tell which is which..
Now, onto your question. I haven't got a BMPCC, however I have extensively graded footage shot raw from that camera and used it in a project mixed with 5d3 raw. I can tell you a few things.
- In terms of resolution, the upscaled 7d RAW looks marginally worse than a native 1920x1080 frame, however it still looks fantastic. Nothing to worry about there.
- Both the 7d and BMPCC suffer from similar amounts of moire in RAW, maybe the 7d a fraction more.
- The dynamic range from the BMPCC really shines above and beyond the 5d3. I also find I can recover more dynamic range from the 5d3 raw compared to the 7d raw, so that's definitely something to take into account.
- IMO the BMPCC is closer to the 5d3 (but as I said, but with greater DR, noisier and suffers from mild moire)
For the difference in price between the BMPCC + Speedbooster compared to what you can pick up a used 7d for, it does make sense to get a 7d (especially if you're basing it on how many pixels you get for your buck). However, I would say the incredible dynamic range of the BMPCC puts it into another league.