To give more detail, go into the Network section of the Sonicwall TZ180, and configure the WAN link. Click on the Ethernet tab, and the WAN MTU setting is there. I did not check the Do Not Fragment box.
Just thought I'd add that the DPI fix resolved the issue for us here...mac workstations behind Sonicwall had previously had download timeout problems and are now matching the speed gotten when directly connected to the cable modem. Thanks to the posters