This has been a difficult query to answer through my usual routes, so I'm hoping this finds a domain expert in the likes of Spex or similar.
I use a lot of R series and RIO devices for test and measurement systems. Mostly, the requirements for these are in the analogue domain, with relatively low speed requirements (10, 20 MHz) in the digital domain. My current favourite device is the sbRIO 9636, which is perfect for my sort of industrial processes - I can integrate it into a rackmount system without worrying about bulky cables, and usually it's able to do most of what I'm hoping it can.
I have a new process coming up which will require a greater channel count in order to accommodate analogue and digital DUTs. In addition, the digital clocked outputs it will have to measure will be more like 50 MHz max. Because I'd like to stick to the sbRIO if I can, my ideal solution would be similar to a 9626 with a 9693 with additional C-Series modules, but with access to additional DIO on the RMC. And yes, I'm aware of what that system will look like (without the need for a custom RMC 
) I'm pretty sure that even at RMC digital inputs, I can probably get to 44 MHz (with no headroom for Nyquist, which isn't so much of a problem). I know RIO devices often quote 80 MHz as the input speed, but I'm also aware that most advice seems to suggest applications beyond 5 to 10 MHz carefully consider timing.
Previously, I discounted using a cRIO with 9403s for the high speed digital because of uncertainty over the channel-to-channel skew and propagation between the input pin and the FPGA. If I could be guaranteed a fixed latency between pin to FPGA with a finite but small skew, I would be comfortable with that, as I care most about missing edges and synchronisation between mutiple inputs from the same device. These days, especially with the newer Zynq chassis making them more attractive, it's worth revisiting...but still those questions remain about channel-to-channel skew.
For what it's worth, PCI/PCIe devices are out due to a compatibility issue with other hardware. USB devices are out due to a lack of robustness within my test environment (whether due to lack of strain relief on most USB connections or because Windows likes killing USB devices from time to time). I can't justify moving to FlexRIO due to cost either. With access to a variety of R series and RIO devices, I know I can benchmark some of this, but it'd be nice to hear from someone vaguely in the know.
So, after a long ramble, my questions:
1. What's the channel-to-channel skew like on board level IDC DIO for sbRIO devices (4x on 9626, 28 on 9636)?
2. What's the channel-to-channel skew like on C-Series modules through an RMC (e.g. a 9403 connected to a 9693)?
3. What's the channel-to-channel skew like on C-Series modules through a cRIO chassis (e.g. a 9403 through a 9068)?
4. Are there any other options I'm missing?
I'd be happy to explain more about my process requirements away from the fora, if helpful.