I need a new streamer.

[maxijazz]

How significant is the improvement over UPnP?
What's exactly better and/or different?

I also find UPnP much better than Roon (both RAAT and AIR).

It is always very hard to quantify differences in sound. And the differences in sound might depend on differences in components in system (i.e. speakers), setup (i.e. how speakers are located and their interaction with room), power (electricity).

Leaving my above disclaimer behind, let say that english is not my native tongue and i would be in trouble trying to describe sound even in my native language.
What i can say is:
1. in my opinion Devialet is light in midrange, some might say it has thin midrange, but to me it is light on timbre
2. for me UPnP on Devialet is 20% better than Roon on Devialet - better are: fluidity, softer transients, softer midrange, less digitus/edginess in midrange band
3. improvement of Roon Bridge on Rpi4 over Roon on Devialet is 100% - better are: fluidity, fuller midrange, much less digitus/edginess in midrange band and most important, rich timbre

So i would say improvement of Roon@Rpi over UPnP@devialet is "small" in fluidity and edginess, but "significant" in timbre richness. 
How much is "small" and "significant"? I guess it depends on speakers, mostly. But even the "small" improvement was felt immediately. I did not have to search for it or listen same song many times. I heard it all immediately, across all music genres. It is impossible to miss the change. That is why i provided big percentages above.

PS:
Please note that since i use Genelec powered speakers, i have used, mostly, Core Infinity card of Devialet. Hence:
Roon Core -> Devialet's CI (Roon Bridge + UPnP) -> Genelec powered speakers

After i added Rpi4/Pi2AES 2/Shanti i don't use Devialet at all:
Roon Core -> Rpi4/Pi2AES (Roon Bridge in GentooPlayer) -> Genelec powered speakers