Connecting high speed DDR ram to an FPGA is not a big deal.
I was under the impression that the kind of memory latency necessary (extremely low) is only possible with the most expensive types of DDR chips.
Let's put it this way: if you actually need external memory to run SCRYPT as used in Litecoin (you don't), then yes it is going to be a total turd performance wise.
It doesn't matter what chip you put out there.