There are a lot of Shaw users having issues with the XB6 in bridge mode:
I'm not sure what the actual modem throughput is on Shaw equipment (Shaw doesn't seem to publish that information in a readily accessible format), but it appears that the Cisco will do up to 300mbps, and that if you go up to 600mbps you need to upgrade to the XB6.
I'm not clear on whether that's based on speed limitations of the modem, or speed limitations in the wireless router.
The best modem test is a short piece of Ethernet cable. Whatever speed you get hardwired directly to your computer will indicate your modem speed (based on your subscription speed of course).
The above all presumes your actual computer can do +300mbps wirelessly ... which many pre-2011 to 2013 computers can't.
I've got a bridged Cisco as noted earlier, and with my new Google mesh rig I'm getting 330mbps wired, and 280-310mbps wirelessly ... so the bridged Cisco (modem) will do 300mbps easily.