Hi Lilian, thank you for the in depth explanation. Just wondering which exact part of the VRM heat sink will interfere with the stock fan on the L12s? Would it be possible to adjust the fan a little to clear the VRM heatsink by any chance?
Anyways, you should expect pretty good thermal performance with the L12S even with a slim fan, the only problem being the slim fan can run pretty loud under load. The actual thermal performance would of course depend on how much you overclock it and how good your CPU is, but 4.7GHz or even 4.8GHz (if you can somehow fit the NF-A12x15 in) should be totally fine for daily use.
Now comes the dumb part - I'm sure you have enough common sense to NOT try this yourself - I managed to snap off the heatpipe that connects the two-part VRM heatsink, and only putting back the part over the m.2 slot (which is necessary if you want to install the m.2 drive at all). I can now install the L12S with the stock fan, and the thermals are now superb. I was running the 8700k stable all core 4.8GHz at only 1.18v ('Lucky me!', I thought too soon, see below), and it never runs past 57 degree even during extended gaming sessions, averaging at only 55 most of the time. I was able to past extended stress tests and benchmarks with this voltage/clock combination, and also a slight OC on my 16G Vengeance LPX from 3200 to 3333 @1.35V, same timings.
That is, until I shorted my motherboard, CPU and PSU by wiring the power button wrong (yes, this was my first ever PC build). I'm pretty sure it has nothing to do with me breaking the VRM heatsink... Or am I? It definitely had nothing to do with undervolting. Putting this aside, the L12S provides excellent cooling for even an 8700k inside the LZ7, and for more powerful CPUs I'd pick it over the new Shuriken even with compatibility issues and the slight price premium.
A harder to implement but probably the best alternative is, as @K888D suggested, to sand 2mm off the VRM heatsink on the ASRock board if you have the tools.