Okay, so I just edited the registry on my device to add the following value:

HKLM\Comm\Cellular Line\Parms\Tcpip\MTU [DWORD] = 600 (dec)

then restarted my device.

I then attempted to connect to a web site I administer and logged the network traffic. I noted that the MSS (maximum segment size) was specified as 1460, so the registry change did not seem to have any effect.