Addressed an error in the sampled rms estimate software used in the response self scanning. The updated results are below. The C board is closer to expected. The high floor still needs investigation. The A board response has a tilt to it and is less flat than expected. To me this seems to indicate either a termination mismatch and/or poor ground on one side of the filter. Unfortunately I need to take the board stack apart to evaluate and address - which if I had used the gold plated connectors would not be a big deal. The large headers on the A board are gold and are amazingly smooth. The pass through headers on the middle B board are tin plated. With the insertion/extraction force involved I get a little nervous with the two layer boards and the synthesizer QFN and where it sits on the board (fracturing working solder joints). I'll defer this until I get some more measurement mileage out of the existing configuration. Pop for the gold plated pins - its worth it.
While on the topic of mistakes, yes the C board filter response is too wide to provide effective image rejection at the A board IF directly (I'll admit it - I misread the data sheet, wasn't closely looking at the horizontal scale. Some of the other parts considered for this stage had sufficiently narrow bandwidth to work at the A IF, however, they were in the 800-900 MHz range. I got greedy and didn't watch the details). Will need to work through the non-instrumentation application options from another IF stage to external cavity filters.