Missing power 440?

Whoa! 3mm is .118 inch. 3/32 is .094 inch. And I run .020!
At 3mm you are more or less at the bottom of the lifter. You need to be more or less at the top of the lifter. The .094 is too heavy for me, but a lot of lifters are designed to run at .080 or therabouts.
The higher you intend to rev it the closer to the top you want to be. I rev 7000 or a little more, so I have minimum preload, so that if the lifters do pump up, They cannot send my valves into the pistons . The penalty for this minimum preload is more frequent valve adjustments, as the parts wear, in the beginning, of their lives.
For you, without adjustability, you don't want to run that close to the top. Further more with that size cam, there is no need to rev 7000, and therefore no need for minimum preload.However, I see two things ;With that much preload, You better have real good lifters cuz there is a very real possibility that the valves may have kissed the pistons, or even that the lifters sent the springs into coil-bind. Your cam is spec'd at .484 lift, right? but if the lifter starts pumping up for any reason, it can add up to those 3mm(.118 inch). Now I am certain that would be a very unlikely situation. But poor low speed performance and the flat spot at 3000 to 4000 could certainly be pump-up. Not likely; but could be.
Here's a bigger question; what if some lifters had run out of room down at the bottom, and were actually keeping the valves open. The results of that would be low compression, and a leakdown test would show a large amount of leakage.And this condition would certainly lead to poor low-rpm performance. After 4500 there may be insufficient time for this leakage to be perceived by the engine so it runs pretty decent. In the 3k to 4k zone the engine is transitioning, from low efficiency running with that cam and running up to peak efficiency, at peak torque.After peak torque she starts to make power.
So in addition to dialing in the rocker arm geometry,I would have to recommend a Leakdown test to prove no valves were bent in this troubling time, and I would be checking the bottoms of every lifter, and I would be checking every spring for proper pressures.While the springs are off you can check the tops of the guides, and the seals, and every pushrod socket in the stamped arms. Oh heck, just check everything.