I'm seconding the partial tail. Let the hole in the air behind you close more gently. While you're building it, build in pannier capability and have a secure, weather-resistant trunk. You'll pick up speed and utility.
Allert gained a mountain of fuel economy in addition to speed with his full-on, no-limits FFW scooter. He changed his gearing twice if I recall, and his top speed is now way beyond the usual 125cc region. If I'm reading his website correctly, his streamliner will now hit 85mph, that's plenty fast. I reckon adding a pannier-tail to your bike would get you comfortably into the 60s without forcing you into a full tuck to get it.