I don't think it's fair to say that your board achieves greater MH/s or efficiency. The board design should not impact this, and both of our boards use the same FPGAs. At $/FPGA, our X6500 has you beat ($290 vs. $460). In addition, nothing is stopping an adventurous user from running your code on our board.
I partially agree.
According to your post X6500 requires 17W for 250 MH/s, that is 50% more than 8.5W per 190 MH/s. This difference could be caused by less efficient voltage regulators, but also by the FPGA design, which is of course not board dependent.
At about 200MH/s the FPGA reaches internal limits. Power supply is critical and makes out up to 10% of performance.