the newer fleet starting with G-BADP onwards had more powerful engines JT8-15A's
Not so. G-BADP & BADR were the first two that were 737-204ADV, but were delivered with -9 engines, like all the earlier ones. My recollection is that occasionally one , or even 2, JT8D-15 were fitted for short periods. The first 737 delivered to BAL with -15 engines was G-BAZG.
The -9 engines were eventually all modified to -9A standard, as I recall; I think the only difference was less smoke, the thrust was the same. What was -15A? I don't remember that designation.
Anorak off!