You're trying to compare two individual fighters. You simply can't do that. The statement of mine that you highlighted in bold was a generalization on weight classes, not a comparison of individual fighters. Furthermore, Cory Spinks and Kermit Cintron have both fought the majority of their fights between 145 and 155, so I obviously wasn't talking about them.
I ABSOLUTELY agree with you, so let's get that out up front.
If you want Kermit Cintron to punch harder than someone legitimately a weight class above him, a multiplier is how to do it.
Let's assume Cintron is a welterweight at 147. If he has 95 punching power and welterweight carries a 1x modifier, his punching power is 95. The 160 lb. middleweight class carries a modifier of 1.1x. Therefore, a 160 lb. middleweight with 85 punching power would be about equal to Cintron in punching power (85 * 1.1 = 93.5) but a middleweight with 75 punching power would be significantly weaker (82.5).
However, if you then go up to heavyweight, it would carry a 1.3x modifier. So, a heavyweight with just 75 punching power would punch with an equal strength to Cintron (75 * 1.3 = 97.5).
This allows for an accurate portrayal of power not only between the weight classes, as it is now, but also WITHIN the weight classes, which is what you want.