Hi vilas,
The same engine and wing is on A319, but there is no flattening around FL 250. The OEI ceiling and weights are identical for both variants around 60 tons and lighter, but the performance of the A320 is better at higher weights.
I think the reason is because the maximum thrust available of the engine is increased on the A320 below FL 250, so that at maximum take off weights, they have similar performance.
Above FL 250, they have similar maximum thrust available.
OEI A319.jpg