Well, closing the intake valve later would actually increase intake tract reversion, and should help the top end. However, the exhaust reversion that comes with a lot of overlap dilutes the intake mixture and causes a week bottom end as well. With your combination (and I know Yellow Rose will disagree with me on this), I would have gone wider on the lobe seperation, and reduced the intake charge contamination from the overlap period. That would also equate to a later intake closing event if installed with the same amount of advance, which would help top end power, but the higher quality (less diluted) intake charge should help the bottom some as well.
A shear plate is a spacer that fits under the carburetor, and is specially machined to break up the turbulent swirls that choke off airflow exiting the throttle bores. Its seen a lot on high rpm engines that have a large plenum under the carb pad. Air likes to follow a surface, and when it can't, it tumbles which induces turbulence and reduces flow. Short answer, a very expensive carb spacer.
