Hi!
This is INCREDIBLE!! I’m amazed by the speed!! It’s like 10 times faster than the CPU version!! 
You are amazing!! …first the multi bounce Skylight and now this?! I hope this will be the future!! Thank you !!!
I did some testing yesterday (discovered your post too late! ;))…
This scene had almost 5000 meshes, HDRI Skylight, Directional light, Point lights and emissive too!
Took 22 minutes (!!!) on a GTX1070 /lightmaps from 64 to 2048/
I had other tests too and run into some errors…
In this scene I run into a “out of memory” error just before the tracing… I had to lower lightmap resolutions… What is the memory limit?
I had a scene lit by a HDRI. The result was VERY nice (detailed shadows) but I had white spots close to corners (will post screenshots in a few hours…) …maybe too few samples?
…and there was another one which turned out completely black… but I will check that again!
Oh I did another test this morning with an older lighting test scene from the forum and I got some strange results (will post later, didn’t have time this morning)… I had some lighting appearing which I couldn’t “trace back”… was not looking real…
BUT overall: IT IS AMAZING!!! And SOOOO FAAAAST!!!
THANK YOU!!!
ps.: What kind of scene/lighting tests would help you ?
/edit/ I forgot to mention the progress bar! It’s so much better than the original (non existing? ;)) one!! …and even has an ETA!! So cool!! :))