I would vote to upgrade LimeSDR PCIe to a small Cyclone V GT which has 5.0Gbps transceivers.
And RIFFA 2.1 Open Source PCIe IP has been tested with Cyclone V GT.
So no Xillybus expensive license fee which is added to the final cost of the LimeSDR PCIe.
5.0Gbps transceivers in Gen2 4x = 18Gbps