I'd have to dig into the source code of that module. It makes sense to not raise it artificially higher than the requesting host. I just didn't expect them to have that logic. IPTables usually blindly does whatever bad idea I give it. :-)

Increasing the value would cause problems. If the remote end says its MSS is 512 octets, either it doesn't have enough memory to receive larger packets, or its link MTU is small enough that larger packets will always get dropped.