Flew DFW-LHR in 777-300 J a week ago. Differences I recall, vs 777-200 F last month:

No amuse bouche in J
No soup course in J
Salad course served same time as appetizer
Pasta entree option was pre-made and didn't offer any options
Entree I had (curry) seemed smaller, but that could be bad memory
Dessert did not offer the "specialty sundae" option (edit: this is the 2nd sundae they have in F; the "regular" choose your topping sundae was available in J)

Breakfast was identical, I think

The inflight snack selection was much much better than before. Sandwiches, pastries, fresh fruit, etc, in addition to the normal junk food.

Meal overall was fine, but not quite the dining experience that F has been lately. I was served first, as I should have been, but we rarely saw the crew after that. Cleaning up my trays took forever though.

As for the seat, there is no comparison to the old J seat, or, frankly, any other J seat I've ever been in. As a J seat, it was fantastic. It is obviously not as roomy as F, but very noticeably so for taller folks - if you are 6' or above, you may have issues fitting. Very tight in the foot well. Tough to get out of the seat unless it is slid all the way back. All of the other extras to F really make a difference, the bed topper, the extra pillow, the 2nd blanket, etc. I really missed them on the overnight flight.

I was in 3A and there is indeed galley noise in row 3 and 4. Really no different than row 3 or 4 in F on 777-200. I would think that 3D/G would be the best in terms of noise. They are slightly further forward, and a little tucked in from the galley.

Overall I thought J was absolutely amazing, and I would be thrilled to never travel in another cabin, but I think there is enough difference in seat, soft products, service, and food that F is still well superior and worth an upgrade or more miles if you can pull it off. And that's to 777-200 F, as I've not even been in 777-300 F yet.