I have experience on 2 types which have FOD screens and both suffer around 8% degredation when they are used. One is the A37 Dragonfly (which has retractable screens) and the other is the S61 helicopter. The ones on the A37 are linked to the undercarriage and cover the inlets when the gear is lowered and there is weight on the wheels. (You can also lower them manually i.e. before a strafing pass). The S61 had FOD guards fitted for summer use only as they can ice up in the winter (as can those on the A37). Hope that this helps!