Hello, I have just made an account after seeing this thread to test out the Wi-Fi for this version of Desmume with a friend of mine. I apologize in advance if this is the incorrect place to post this to, but I've found some results after doing some testing with him (Emu to Emu). 

The game which we were testing out was Mega Man Starforce 3, specifically the Japanese version of the game because for some reason the English version won't let us see each other when searching for a match. With the JP version, we were able to find each other fine without many problems (Sometimes it would take a while, but most of the time it didn't take too long). Once we were able to connect to each other, the Wi-Fi didn't DC at all and we were able to rematch each other afterwards without many problems either. But, the actual matches were extremely slow and choppy, making them truly unplayable.

Because of this, we decided to mess around with some of the emulation and 3D settings while also turning off the framerate limit so we could get some extra speed. These were our settings for both of our emulators:
http://puu.sh/BA67r/c4fa62b2de.PNG
http://puu.sh/BA67x/a31ca52255.PNG

Once we've applied these settings, we were able to get the game to a far more playable state, although is also quite a long way from perfect. But it is playable enough for me to dodge and block some attacks just in time. Though if you use bomb/nuke attacks in this game like my friend did around halfway through the game, then the game will slow down a little and become somewhat choppy. Here is a video showcasing our netplay match:
https://streamable.com/k6fdm

So far that's the best we've got to making it playable. We also tried fiddling around with the sound settings to see if we could get an extra speed boost, but all it really does is make the game far more unstable when on Wi-Fi, since it will either not find my friend at all or DC randomly. I hope this is of some use and I don't really expect a solution to it anytime soon, but I'd be really thankful if there's anything which can make the game more playable.