I had a go at doubling up the focal length of my William Optics via a 2x barlow a couple of nights ago. Unfortunately this does increase the exposure time by a factor of 4 but it givesme a better view of the cluster due to its relatively small size on the Canon.
Canon 300D on autoguided EQ6
William Optics with 2x barlow (EFL 1090mm f/14)
4 x 5mins @ ISO1600