Okay, here are the benchmarks! I've included my own Pentium J4205 (Apollo Lake) results where available and calculated the performance difference. Please note that the J4205 results are from Nov 2016 and used a different version of Windows as well as a much older graphics driver.
I observed the J5005 clock frequencies during an MT run of Cinebench R15. The CPU starts out at 2.7 GHz on all cores, but after ~1/3 of the test it settles down on alternating between 2.4 and 2.5 GHz.
It looks like the J5005 is 30-35% faster than the J4205 on average. I haven't done any extensive comparisons of operating frequency, but I'm guessing the J5005 runs at roughly 5% higher frequency. So average IPC improvement over Apollo Lake is probably in the 25-30% interval. Pretty impressive. Using Geekbench 4 for comparison, the J5005 seems remarkably close to the IPC of the Nehalem-based Core i5-760. Not spectacular performance for 2018, but I'd finally call the "Atom" pretty usable. This NUC, with a 256GB Crucial MX100, definitely doesn't feel slow. Single core performance appears to hover around 4 times that of the original Atom N270 (1.6 GHz) released in 2008.
After the table below, I've included URLs to some of the results (Geekbench/3DMark).
Result URLs
Geekbench 3:
J4205
J5005
Geekbench 4:
J4205
J5005 (new result with latest version of Windows)
3DMark Firestrike v1.1:
J4205
J5005
I spent an hour trying to get BOINC Manager to add the project, but it would always fail with a message along the lines of "Couldn't add the project. Please try again later". Yay... Tried two different BOINC versions and even attempted using boinccmd. Got a different error message there and decided to give up.