I'd say 2.5 pro to 1.5 pro was a 3 -> 4 level improvement, but the problem is 1.5 pro wasn't state of the art when released, except for context length, and 2.5 wasn't that kind of improvement compared to the best open AI or Claude stuff that was available when it released.

1.5 pro was worse than original gpt4 on several coding things I tried head to head.