I have difficulty cleanly attacking lower register notes with my f attachment .547 bore trombone using a 1&1/2g mouthpiece. Could I expect that a larger bore bass trombone would improve these notes?
A larger bore bass trombone will let you put more air through the horn in a manner that will let you more efficiently produce low register notes. (Lots of slow air).
In my experience, this certainly helps, but will not magically solve all problems. It may even introduce a few of its own, especially if you're playing it for a long period of time, without specific bass oriented lessons.
I organized a trombone quartet to play one piece for my college senior recital, and played bass for that. It certainly made a big difference in the low register (anything lower than E) in the tone department. The articulation was better than a bass mouthpiece on my .547 bore.
However I noticed that in terms of articulation, not quality of sound, my cleanest articulation was on my normal .547 setup with my normal mouthpiece. Putting a larger mouthpiece on it simply muddied things, although made it easier, lip-wise to produce the pitch.
Your mileage of course will vary, but it might be worth borrowing a bass for a few days to see if it seems to make a difference. That would also give you a starting point as to what to look for when it comes time to buy.