To answer your question, I would say there is a speed gain, around 10%.
I get just as tired, because I am pedalling just as hard.
Some of the fairings I posted pics of can carry cargo, even rocks and dirt for trail building projects. Frame mounted fairing can carry 100 pounds of weight.
The fairings I have built offer crash protection. I've been "right hooked" and managed to turn it into a "Pitt maneuver", where I left the car facing traffic the wrong way.

Weather protection is excellent, and I ride in 15 degree farenheit cold, that no one else will ride a bike in.
I especially like the way the fairing keeps sleet / freezing rain off of me.

But as I said, Florida people test rode the Type 5 and said they got too hot. The Type 7 and Type 9 were designed with smaller fairings so the rider stays cool.

There is no way anyone can invent or build a fairing that will be perfect for all the World. Northern climates will inspire bigger fairings and velomobiles, while more at tropical lattitudes people will view them as "sweat-boxes".