I haven't noticed much difference between fly and batch sparging with reference to efficiency.
If anything I'm getting more efficiency but at the same time I have how fine I crack my grain etc.
The reason I changed is effort. I find it much easier to simply fill up the mash tun and set the run-off to the speed I want, rather than making sure the inflow matches the outflow etc.
What made me change ? Trying to get the grain bed up to mash out temp is what got me going down the batch sparge road. Now at the end of the mash I simply fill up the MLT and let it settle, drain and repeat. Much easier.
Beers,
Doc