Been using one on my tz250 since 01. Also use one on the current moto 3 bike I make - GPtech bikes. Creates more down on the front wheel. I actually noticed a difference it made in the last turn at willow. Ed Sorbo used one too.
In response to the OP question...apparently by the rules it is not...my interpretation anyways. Quote from the rulebook...
I have read several articles in the past that stated it can adversely effect cooling on the small bores...don't know first hand.
Except for the bikes that don't come with bodywork from factory, like sv650, where there is no restrictions on bodywork.
Has fender but no full body. Rules say no restriction, even in SS. We can use any bodywork out there.