I'll probably get flamed for this but how about a 135 vert? It is smaller inside but has more trunk space with the top down than either the G37 or the E93. And, it's as fast as the E92 and faster than the E93. Not sure about the G37 but I would think it would be faster than that too considering it is a hard top convertible.
2014 EBII 435i xDrive M-sport
Gone: 2011 TiAg F25 X3 xDrive35i M-sport
Gone: 2008 TiAg w/Coral Red E88 135i Cab M-sport