It is definitely true that a windscreen will yield more range improvement (10-15% for a good, well-positioned screen).
Fork deflectors could work a little, in principle. I’ve used wheel disc covers for about 3-5% range improvement and wouldn’t expect more benefit than that. But you’ll have to design or adapt some and test this yourself to know for sure.
Right i should try it out. I got a pair from
http://buywitchdoctors.com/ that should've worked for my old Empulse TT, but never got to try it out on that.
Not sure if the same pair would work on my 2018 Zero S (as the forks on this have a smaller diameter). It could work if the fork hooks are small enough to fit.
Also i use a Puig Rafale windscreen and i've positioned it low enough that when i tuck in fully the air goes just above the helmet. Seen efficiency gains for sure, but havent measured. I'll report back if i get to try the deflectors.
Doubt it would be good for much. The most unaerodynamic thing on the bike is your upper body...
Would it help if it also deflects around the battery box and not just the legs?