Potential gains on tuning depend on the a/f ratio associated with your runs. For most of the naturally aspirated set ups I've seen, each point richer than safe optimum (around 13:1) gained about 8-10 HP. So, if you were around 11.5-12:1 a/f and you leaned it out to around 12.5-13:1, you'd likely pick up 8-10HP at peak.
Grn92 - I know what you mean about apples to apples. Measuring the actual impact of any one change is definitely an art/science combined. It's really hard to do effectively and accurately. But all I have to do is hold that fan/clutch assembly in my hand and I know I'd rather not be asking the engine to spin it. But who knows. When I went back to the dyno to tune, I had removed the stock HO headers and added FRPP shorties, and I (for grins) dumped the dino oil, and added a crankcase of Amsoil. Ambient temps were almost identical - 40F and low humidity/high pressure. We lost 3HP. I think the moral of the story is that most dynos aren't able to achieve repeatable results with that level of accuracy. I've got a feeling, notwithstanding their advertising to the contrary, that if they're within 5% repeatable, you're doing great. Heck, I've seen tests where one click tighter on the tie down straps added 10HP to the readings. Well, with that kind of variability, when you're looking for 3 or 4HP, it's kind of like looking for the proverbial needle in a haystack. And they keep moving it in the haystack.