Sorry, I've been meaning to make a post related to this. We have been working with the OP to determine the optimal configuration and board for this application, as we have a few to choose from.
If you could include a table showing how much (LP)DRAM and SRAM (or low-latency alternative) can be hooked up to each FPGA, as well as the resulting cost, that would be very helpful. Thanks in advance!
The best thing to hook up to the FPGA would be a hybrid memory cube +$500. This would get you the same level of memory performance as HBM memory. The other nice thing, the HMC has a silicon memory controller on it along with some basic logic functions that can speed up certain applications (xor, and, or).