Have a baglux cover on my GSA- it fits quite well but is very stiff when new and took time to mould to the tank- its now fine.
As regards the bag- I have the standard alpha bag and it can swallow a lot of stuff- as I only use it as a bag for transport on hols for non-valuable stuff as the rest gets locked in a pannier.
I dont think it really requires the rucksack straps- they do take up some room. As regards bulk- dont really notice it when on the move- between the arms that is. I am carrying some extra 'luggage' on my person (if you get my drift) but if I were a little smaller I wouldn't notice it. The clocks and my GPS don't get obscured, and the handlebar controls on full lock don't really bother it.