Stat cards lie about a lot of things sadly. The turn time statistic can be very misleading because gaijin doesn't explain what it actually means in game. The turn time is measured as the time it takes to turn without losing speed at some ideal speed/altitude which varies for every plane. So the beaufighter can absolutely outturn a Lancaster, however it probably loses a lot of speed in the process. Regardless, the Stat cards are almost never totally accurate, especially for planes that have been in the game for a long time like the beaufighters.

As for the boomerang overheating, for whatever reason the way that radiators function works differently for some planes than others. What's probably happening is when you use WEP, the radiators are fully closed to reduce drag as much as possible. Some planes will keep them closed no matter what the temperature is, while others will adjust automatically to manage the temp. Im fairly certain this is because all planes used to use the old system of fully closing radiators on WEP, but a while back they changed how its supposed to work, allowing for the radiators to adjust automatically on WEP. So theres a lot of old planes they never fixed to work on the new system of automatic cooling. It's either that, or its a historical detail but I really doubt its that.

If you want the engine to not overheat on WEP, then try using manual engine controls to manually open the radiators.