You should get the same "tier bonus", but the "cabin bonus" will be lower: 25% extra Avios in business class with an AA flight number, regardless of booking class, rather than 50% for I (or R) with a BA flight number. The difference on a TATL return itinerary can be enough to make it worth booking the BA flight numbers. Check using the calculator linked to above.
The difference is more significant on more expensive, more flexible fares, as you'd get 150% cabin bonus with a J, C or D fare booked as BA.
its true you will lose some Avios flying AA metal however you will be more comfortable, better fed, better served and have access to T3 lounges. For some, myself included it’s a small price to pay.