as an AA EP or CK for about 8 years (and lifetime plat) before i moved to london permanently, with frequent trips to the US, my experience was this:
first year to qualify at BA GGL my first year was super easy. a handful of premium long haul, with loads of domestic US flights in F. -- the key here was this was BEFORE AA fixed their fare buckets. i would regularly bear the JFK-PHX-SFO to obtain 270TP in F which now reads J.
so it is a bit more difficult with domestic travel. however i will say that as a BA GGL in the US, i have been treated better than i ever had been in all my years as an AA frequent flyer. the free lounge access (as opposed to some years of paying for admirals club access) and similar bag allowance still remain, as does access to the AA explat phone line when i call for any issue on an AA flight.
downside of course is never being able to use any of my 500-mile stickers, and no SWU or being able to upgrade on AA longhaul since i purchase them now via BA.
you always earn your BA avios and TP on AA operated and marketed flights so long as you provide your executive club number in the booking.