Marcus,

I don't know how much better Litz can do in your case but  the winding
length is also important for efficiency. If I remember correctly, best 
efficiency
(power transfer) is achieved when the winding covers the entire length
of the ferrite rod i.e. the coil uses as much length as possible of the 
concentrated
field . Unfortunately, this does not coincide with maximum coil Q.

73
Johan SM6LKM