the dedicated I/O to a standard PCIe mechanical connector as well as access to the PA6T GPIO lines are a nice addition. i actually am planning on using this with my X1000 to build an HF receiver (RF stage, A/D conversion, etc. on PCB attached via the 'xorro' connector, SDR via the 'xena')
I'd be too scared to plug my own PCB to a $3000+ system to be honest. Especially when any damage resulting from the custom HW would most certainly be outside of warranty repairs.