I'm not sure I understand the question, but a large fraction of the
total power is typically in the main lobe of a Yagi. You won't increase
the power in the main lobe significantly by reducing or eliminating
other lobes, because there just isn't much power there. If you want more
power in a narrower range of directions, you need more directionality,
which means a longer Yagi, stacked Yagis, or some other type of antenna
which will probably be larger. The methodology for and tradeoffs
involved in increasing directionality are well known. And because Yagis
(ones not having lossy traps or loading components) are very efficient,
directionality and gain are inextricably linked.

Roy Lewallen, W7EL