Mine resembles a Batwing, but is the Radio Caddy Fairing from the
http://www.trikeshop.com/shop/product/49/radio-caddy-fairing/ .
It additionally, to a Bat Wing, has a bump out on the top side that I would have thought might add to instability. I have sensed no unusual or unexpected wind behaviors @ 15-20 mph from any direction. Any stronger and they'll push anything around. I've had mine to triple digits with no apparent fairing issues.
I feel as long as it's mounted level and plumb it should be fine.
If a fairing is not level it will tend to want to "dig" in to one side or the other at higher speeds. Under most wind or speed conditions you'll be fine. They generally provide much better wind management then a windscreen alone and occasionally wouldn't be missed with high ambient air temps.
The wind screen is from
http://www.ceebaileys.com/harley/flhtssc_ws.html which protects very well and reduces drag a bit.