I would expect the impact to be small -- Apple releases new hardware on a very predictable schedule, and this kind of news is a bit esoteric to influence the casual buyer.
Tell that to people in the market for pro-level Mac hardware.
If the information of the new product provides consumers even a reason to wait that could cost Apple dearly, this is why they are notoriously secretive.
Most customers are not even aware of possible model changes when they purchase. A reasonably technically adept friend of mine just bought a new iMac the other day. When they told me, they were very happy about and and I had to avoid noticeably wincing. If I had known before the purchase i would have advised them to wait for the upcoming redesigned models with the M-series chips instead of the old stuff. I think that this is how most customers approach purchases. They don’t pay attention to rumors and news about newer products.
But I think Apple's secrecy is more about maximizing the marketing impact of announcements, and making competitors work harder to predict product directions.