I thought about this as well. The water velocity might be a little more effective as it drops from a higher elevation. Then again unless there where a funnel under the tray there would be too much turbulance to enable this.

Sometimes I add some half bricks to reduce internal tank volume as this tends to retain the head of water height. Adjusting the float level turnoff lowers the general level. Gravity is free and benificial to work with so why reduce it by lowering the tank level?