So I was checking in over at the Daisy Gate at Gateway to Air guns and saw a post about increasing the performance of the Daisy 25.  User PoolDoc posted the topic.  I am not a model 25 expert but wanted to pass this info along.  The original thread can be found here.  I will attempt to convey the information here but encourage Model 25 knowledgeable people to chime in in the comment section below.
The upgrade comes from replacing the factory spring with a stronger spring such as the ones found in the Daisy Model 40 number 111 Red Rider. These springs can be found at various websites such as JG Airguns . I have seen listings on ebay as well. The ebay listings look to be new springs so I am assuming the sellers sourced the springs from the factory spec.s. According to PoolDoc the 111-40 spring measures 9-3/8" long and the wire gauge measures at 0.089". To source the spring we would also need the OD and number of coils. The spring is too long for the 25 so PoolDoc said he had to cut off 5-6 coils. He then heated, flattened, and ground the cut end as flat as possible. After installing the new spring the gun was noticeably harder to cock but "not too bad". According to PoolDoc the spring upgrade increased FPS by more than 82 FPS with standard BBs. I will update this post if more specific information is made available like an independent spring source, more technical data, etc...