For some reason Garmin does not enjoy as good of a repuatation on the sonar side as they do on the GPS side. I have no complaints on the performance of my 4208 with only a 600 W transom mount transducer. I was really careful about installing the transducer in accordance with the template and maybe that has helped. On a very calm night, I can see bottom while swordfishing in 1400'. But I'm usually looking for bait in the water column much closer in and I'm not sensitive to really fine separation between fish and structure, which, I'm told, is where Furuno shines.
As far as I know, Garmin only uses XM Weather and not Sirius. I don't know that it makes any difference now that the two have merged.
Rob