You mean a wider cone right? I guess depends which dimensions you are talking about. according to Gordon Jennings a wider (more degrees, steeper) gives a slightly stronger wave, so higher peak power, but cuts out after the peak, and no more than 20 degrees should be used (a flat plate!?! thats like 180 degrees!?!), and vice versa; weaker wave, lower peak power, but power curve extends past it, and no less than 12 degrees.
I am glad I found this thanks!