Just to report back, I had the following routing in the end:

JFK-LHR-MUC on BA 747-400

Pros:
-BA Galleries lounge at JFK was great, but a total zoo.
-Easy connection at T5
-Galleries Lounge at T5

Cons:
-Abysmal security lines for BA premium & status pax. Much easier and quicker to go to the normal security line in the main check in hall
-Horribly uncomfortable seat, I actually might take a Ryanair slimline non reclining seat over flying a BA 747 again (though I usually love the aircraft itself, just not on BA)
-Tiny IFE screen which cut in and out throughout the flight


Return was meant to be a BA 787 LHR-EWR but it was cancelled due to the aircraft being taken out of service. BA was woefully inadequate during IRROPS. BA.com wouldn't allow any rebooking online, and when the automatic rebooking finally came through it was for 2 days after the originally booked flight. It took 2 tries and over 2.5 hours on the phone with BA to get in touch with an agent, who warned me "You are aware if I book you on an AA flight, it is not a BA product and it is an inferior level of service to what BA provide."

Turned out to be a very interesting statement in the end....

Finally got booked on an AA 773 service but it took 12 hours for the ticket to be reissued, not allowing for checkin, except at the airport, and missing out on an MCE seat.


LHR-JFK on AA 773

Pros:
-Seat was very comfortable
-Fantastic IFE
-Better policing of boarding and bag stowage by cabin crew
-Best AA crew I've had in a long time
-Quickest Immigration/Customs and Baggage Reclaim I've experienced at JFK

Cons:
-Admirals Club at T3 (vs. Galleries at T5)


From now on, unless at a real push, I likely will not be considering BA on any long haul journeys. Their short and medium haul is great, but I wouldn't get in another one of their 744s again. The lounge experience on both sides is great, but not enough to make up for their inferior hard product.