Defiant and raptor do not have the same turn rate. And even still, the raptor vs. fleet escort the turns are identical. It is a myth that the shape means anything in the game.
I can also say without a doubt my ships do not turn worse than they used to, escorts, raptors, cruisers, whatever. They all feel the same, and the numbers bear that out. Of course none of them have 9 in impulse thrusters either.
However, I have heard it suggested that impulse thrusters does not increase turn rate past rank 3 for no apparent reason, yet continues to increase speed all the way to rank 9. This might give the illusion of a worse turn rate when you are constantly overshooting. Your speed most likely went up with the season 5 changes. I know all my escorts run less engine power than before because they were going too fast with the changes. Of course your turn rate is actually still the same in that case, its just a wider arc.
This makes sense, your turn rate may be the same, but due to higher speeds your turn radius is up. Definitely plausible.