Machupo, I think I had success with the buck converter method. My build worked for about a month with superb graphics performance, no crashes, artifacts, etc. The motherboard eventually died, however, but I think that was because either (1) some water leaked from my WC loop, or (2) the thin mini-itx mobo was only rated to deliver 25W at the pcie slot, and the gtx660 was spec'd to draw as much as 75W. I concluded those two options because both the GPU and CPU were still functional. I've since sold the working parts, but decided in the last two weeks to make a similar build all over again (now that I have money again ). I was confident enough in the buck converter that I'll be using it again.
I'm assuming you are using the 330W x51 ac power adapter. Are you using the power board as well, in a custom build, or do you actually own an x51 (chasis and all) and are moding it?
Here's some nitty gritty number and spec crunching if it's helpful to anyone:
Looking at your components, the CPU has a TDP of 77W. The GPU has a TDP of 170W. That's 247W, add at least 20W for mobo and HDD etc, you get 267W. At 19.5V that's about 13A drawn from the AC Adapter, which (if I remember right) is rated for up to 16A. If you don't overclock the CPU, the 330W brick should be within its operating limits.
Lets look at the buck converter and the pico PSU 12V rail and consider those options for a second: The affordable and compact voltage step down device that I got, worked in large part by dissipating energy as heat. I don't know if it was actually a buck converter or not. But my point is that any converter might decrease the power efficiency of your system, meaning you should add some additional headroom for your GPU. If overclocking, you'll have to factor that in, as well.
Now for the picoPSU, which will power your CPU mobo etc. Most pico's take a 12V input. A 12V ac adapter @ 16A (if they exist) would have a rating of 192W peak. That seems to put a single 12V adapter out of the picture. The only pico I know of that takes 19V is their 120-WI which takes a variable input up to 25V. Looking at the manual, the max sustained load on the 12V rail is 6A, and the peak load is 8A. I'm not a bonified computer engineer, so I'm not certain, but it seems to me that the CPU is going to be powered by this 12V rail. That means the 120-WI pico could only power up to 72W on the 12V rail, which means overclocking a 77W CPU is a no-no, and even at that, you probably would want to underclock that CPU if driven by the 120W pico.
My advice: if you have time and lots of money and you don't mind the headache of weeks and weeks of troubleshooting, go for it. Otherwise, if you have a working build, it might be best to stay with what you have or consider a SFF box PSU.
edit: Maybe the buck converter in line with the PSU, and then split to the pico and GPU would give a little more headroom on the pico side. The pico 160XT is rated at up to 8A on the 12V rail. I'd forgotten above, however, that the GPU's pcie slot would be drawing as much as 75W (which could also pertain to the pico). That means the pico 160XT could power as much as 96W on the 12V rail. However, considering the possibility of 75W from the pcie slot, that could mean the CPU+PCIe could try to draw as much as 152W, or 12A from the 12V rail on the picoPSU. Uh-oh.... From what I've read, there is no spec on whether a gpu draws from the pcie slot first, or the auxiliary pcie power connectors first, or equally from each, etc. That means you should expect 75W to be drawn from the PCIe slot on the motherboard. Long story short, picoPSU's are extremely limited. They simply can't power a CPU/GPU combo above 96W, best case scenario.