I'm curious as to why you want to make them brighter? They aren't on very much. They only come on in certain circumstances.
Either way, I don't think there are any options out there to make the cornering bulbs brighter. I think, as TECH said, you can only really look to change the color for a better match.
'11 BMW E92 ///M3 - ZCP and DCT
'15 Ford F-250 - Lariat, 6.7 Powerstroke Turbo-diesel