Good thought - I'm honestly confused on which came first - the damper or the head - I think the damper died the week after the head went on. Both were in late Feb 2010. I went back and looked at some posts. Looks like I was happily seeing scads of boost (more boost than before, in fact - a bit over 15 psi) as recently as April 2010 - so at least at that point, the car was behaving "normally" as far as building boost is concerned - and this was well after the damper and head had both been on for a couple of months. Soooooooo..... I don't think it's anything that got introduced with the head upgrade, nor with the damper. The ATI damper isn't supposed to fail like the OEM. I seriously doubt it's slipping - that would manifest the same way a SC pulley slipping would, and boost would be all over the place. That wasn't happening on the dyno... I just wasn't building more than 12 psi on the dyno (takes longer than the 10 second pull for boost to build beyond that now).
Paul, You can also rule out the head and the balancer. There is no physical condition in the head to cause what you originally described. A stock balancer with some wear might slip occasionally under load more so if all condition were right but usually without the same results as what you are posting. The ATI balancer by its design would make it rather unlikely to slip if installed correctly. The hardest thing determining a solution to your question is the inconsistency you report that it is a gradual increase to your normal maximum boost which eliminates many of the the good suggestion to its cause.