I'm always a skeptic on dynos - I don't really take them as bible truth. Ever. But they are useful tools - and in this case the tool is saying it's way down on power to where I'd expect it to be. Maybe not 400, but certainly 40-50hp (15% +) more than it is showing.
If the cam was degreed at all, chances are it's close. Or maybe the better way to say it is it's not "15%" worth of out. I've never had to move a Comp product more than 4* when the base machining was "good" and the parts were mid-price point or better.
Tuning the A/F may give a few hp here or there but it's not going to "make" this combo. It's close already.
Rings should be seated, but might not be, so it may get a little better as things seal.
This is where I'm at: I think the valvetrain is the problem. Lifters may be floating keeping the upper rpm power lower and limiting the ceiling. It should not be done under 5500. It should pull cleanly (and make power) past 6K with that cam and as-cast 906 heads. It's not.the camshaft has a "recommended" lifter and spring. IMO That is what should have been used. I wouldn't use the stock lifters and weaker springs on anything pas the XE268. That upgrade would have meant spring seat and guide work, plus adjustable rockers. Those would have added $1K or more to the build and the builder may have thought it not worth the price. It's a good engine, just not quite where the "sum of it's parts" should put it.