I used to have films of brewsters passing HO turn 180 degrees level and seeing the data in the film showing nearly zero loss in speed.
Of course every aircraft can turn with zero loss in airspeed, by reducing the load factor during the turn.
The Brewster's performance in AH compared to two other aircraft can be seen in the Doghouse plot below. Along with sea level values for Corner velocity, climb rate and top speed, those values along with the Ps=0 curves shows that it compares very closely with these other fighters, yet they don't receive the same negative attention.
The Ps=0 zero curves show that the while the Brewster sustains energy in a turn better than the I16 it is significantly worse at sustained turning than the A6M2. In a sustained turn, the A6M2 can sustain almost four degrees per second more than the Brewster for what would be considered a decisive advantage.
One thing I think helps the Brewster a lot, is that while the Zeke, and many other aircraft lose control authority at high speed, the Brewster appears to retain full control authority at speed well above its top speed which means it is easy to maneuver when diving from altitude and also at very low speeds, so it can be more deadly in dives and zooms than aircraft that lose some control authority in those situations. That of course explains why the experienced Brewster drivers fly it that way.
However I think it can be seen that the Brewster doesn't stand out in this comparison in terms of its sustained turning ability.
During my flight tests of this aircraft back in 2009, nothing else seemed to stand out as noteworthy. In the years since then, when ever I've been surprised at the speed or energy retention of the Brewster, it has always been as a result of some hidden/retained energy after a dive and that's just a testimony to the fact that it is being flown to its strengths.
The simple fact is that to do well in any aircraft, you need to know it well and fly it to its strengths, in that regard the Brewster is no different to any other aircraft in the game.
Hope that helps
Badboy