An undertray seems to improve airflow through the cowl. I've had one on my Roadster for years now, and the bonus is it keeps road grime from collecting on the rack and crosshead.
The main thing for me is keeping the grime away from the rack and crosshead. It just looked so exposed beforehand. I removed the I've riders to help getting the bolts in, and found out how fiddly it is to refit the lower bolts 😖
