25 cfm is 25 cfm. On an engine making say 2-2.3 per cfm, thats a big gain in power. As far as the small block heads, the gains won't be earth shattering as far as et gain. .2 and 4-5mph would be a reasonable gain, without changing anything else. Sounds like a similiar 406 I did a while back, flat tappet deal. Iron Eagle Plat 215's. Did some basic detail work, it made 580hp. He made 300+ passes on it, and had us freshen it. Added 2.08 intakes, and did a bit more work, still nothing crazy, it picked up 35hp from the additional head work. Keep in mind, they weren't box stock to begin with, when the engine was new. So, from box stock to a full basic port job, there is 40-50hp in them. Obviously with an even bigger intake valve, and some radical porting, the potential is even better, but its application specific.
Frank