I implemented an algorithm for packing UVs quite a bit tighter than Blender does it. The algorithm is quite simple so it took a lot of optimization to make it fast.
It’s up on https://gumroad.com/l/UVShotPacker (paid)
There is currently a project underway to implement a better UV packing algorithm into Blender (https://blenderartists.org/forum/showthread.php?397599-GSOC-2016-UV-Tools). So you might want to wait for that. But it’s not certain when that will be finished.
Any feature requests and bug reports are welcome.