The Yak-15 is probably my favorite plane to fly in this game, but its performance has always been weird to me. The sustained turn rate on min fuel is better than the vast majority of props, including the Yak-3U, of which the Yak-15 airframe is mostly based on. It’s just weird because although the Yak-15 is lighter, props are far better at low speeds than jets of that era and I would expect a prop to be far better than a jet in a dogfight and especially in terms of sustained rate. Is there any real-world data to explain the Yak-15 in-game?
It’s all about the engine.
The biggest performance related problem with early jets was that the engines spoiled up slowly which had a major effect on acceleration.
In sustained close combat where energy and speed is too easily lost an early jet can easily can be defeated by an ‘inferior’ prop who can simply last longer and react quicker at the bottom end once drag and fighting inertia takes its toll on the jets initial speed advantage.
This is why the best tactic even in an agile plane like the Yak-15 is sometimes to boom and zoom, take wider turns, and don’t be afraid to break off before you become a sitting duck - "luckiest is he who knows just when to rise and go home.” (John Milton Hay)
For turning it’s likely the simple lack of propeller and drag profiles of the altered fuselage would be factors in equalling or even bettering the performance vs the original Yak-3U. You are nolonger fighting the torque or the drag of the prop itself and have a different draggy profile. You still have other factors such as c of g and moments of inertia changes but the change of engine would affect more than just the straight line speed.