I've read the SPR review of the NH-L9 and want to know options if it is too risky a choice for the AMD-5800K with socket FM2, esp. if you give it a modest overclock at all.

Noctua says the NH-9Ls are for up to 65W TDP for AMD but has a qualification that leads me to believe that I could use it with the nominal 100W TDP 5800K which idles at about 43W. The qualification is here:
http://noctua.at/main.php?show=nh_l9a_tdp_guidelines
and reads The SPR NH-9Li review used an overclocked 95W CPU and did quite well.

I will be building a new system. I live in a relatively cool climate so meet the below 25 degree C ambient guideline. I am thinking of using the Silverstone Temjin-TJ08E case with excellent cooling along with an ASUS F2A85-M Pro MB.

If folks think my idea is too risky, not likely to be quiet, etc., please suggest an alternative cooler.

I don't own the Noctua, but according to what I read I think you should be fine from a thermal point of view in the well ventilated case you choose. The fan in the NH-L9i is the same as in the NH-L9a and if you're OK with 34dBA at 100% (listen to the recording from the NH-L9i review for the noise characteristics) it should be fine for you.

(For most use cases the CPU doesn't even use close to 100W unless my PSU is more efficient than I thought. I can do some power measurements for different use cases at both stock and slightly undervolted if you want. Surfing the web/playing videos is 50-60W AC, playing Alan Wake takes about 140W AC.)

On the other hand there might be cheaper alternatives for your relatively big case. I'm no expert, but as far as I know you should get both better performance and quieter operation from a tower cooler like the Cooler Master Hyper 212 Evo while paying less. (In Germany the Noctua costs about 40€ while the Cooler Master is about 30€) The Noctua is designed for really small cases and you pay a premium for this.

Thanks, johannes, for pointing out the MP3 recordings which I had passed over when I read the review as I had not listened to them before.
The 34 dBA was not terrible if you don't have to hear it all the time.

The CM Hyper 212 Evo also looks like a good choice. Newegg buyer feedback only faults installing it as a possible pain but otherwise it gets outstanding reviews.

I will have to look into a quiet PSU now unless I could use my older Antec Phantom 350W PSU.