The Supreme is a significant upgrade the Superfly. As Snopro says the Radian 850 is a big upgrade from the older tweeter. More open, sweeter and transparent without any hardness. I do think it's more than just the tweeter. It takes everything that was great about the Superfly to a higher level. The better things are upstream,(amp, preamp, source) the more your going to hear what the Supreme can do.
What's nice about the Superfly and the Supreme is that both can be paired with super components without apology but they also sound really good with more modest components.. I'm using an Atmasphere S30 OTL which IMHO is a match
made in heaven. There is nothing except the speaker wire between the tubes and the 10" driver and only a high quality capacitor and resistor to the tweeter. But I have also heard the Superfly with a Stereo 70, CJ MV52, and Audio Research Classic 30 with excellent results.
If your finanical resources allow would go for the Supreme but the Superfly is also a terrific speaker.