I did pretty much what you are talking about last year--installed the 545s and ran that big honking Garmin cable inside my 1 1/8" Edson pedestal guard. But I have to admit, I didn't install an XM antenna. The 545s sits on the Edson tray that sits between the two bars, not on one of the ones that goes off one bar.
But I did cut the connector plug off the cable to keep my hole size down to something like 7/16". I spliced the transducer cable following instructions of Airmar, the maker of the shoot-thru-hull transducer. They said splicing would be fine, recommended soldering, and they were right, it was fine.
As I remember it--keep in mind that boat projects fade in my memory to a haze of sweat, fret, and gymnastics--I had to splice one of the wires up top before pulling the cable down through the tube foot.
2000 Beneteau 331
Northern Chesapeake Bay
Ambition is a poor excuse for not having enough sense to be lazy ~ Steven Wright