From what I learned so far, it seems that DRAM-less cache-less SSD like the SanDisk Z400s would give the best battery life, although you seem to need some solution to prevent write amplification that comes from the lack of DRAM. This is my theoretical solution to solve the issue.
EDIT It seems that Windows has RAM caching built in, anybody can confirm? If thats the case, then why do SSDs come with DRAM?
If outright performance is the top priority, the crown easily goes to the Samsung PM961 (similar to 960 EVO), hands down.
If the PM961 is too expensive and the DRAM replacement is too difficult to implement, then the good ol' 850 EVO seem to be the jack of all trades. In addition to that, it seems to be less power hungry than the PM961 or any other PCIe SSD for that matter.
However, if PCIe is a must, here's a list of some of the most popular PCIe SSDs, including the 600p and the aforementioned PM961. The graph shows the battery life on the Lenovo Y700 when equipped with each SSD. In term of performance, almost every single one of them is faster than the Intel 600p, thats more than sufficient for the Eve V in my opinion.
Both the Plextor and WD have Marvell 88SS1093 controller, leading me to believe that this controller is the key to a power efficient SSD.
It is probably also worth taking a look at the ADATA XPG SX8000, although I dont have any information on its power consumption. It might be similar to the 600p since they share the same controller, the main difference might come from the fact that the ADATA doesnt have separate, huge SLC cache. ADATA instead implemented SLC-mode in the main MLC NAND, buffered by a DRAM that the 600p lacks. For the results, the (claimed) read and write speed both favors the SX8000 over the 600p.
Best battery life (SATA): SanDisk Z400s (or other DRAM-less cache-less SSD)
Best battery life (PCIe): tie between Plextor M8Pe and WD Black
Best performance (SATA): Samsung 850 EVO
Best performance (PCIe): Samsung PM961
Performance: Z400s < 850 EVO < 600p < WD Black < Plextor M8Pe < PM961
Efficiency: 600p < PM961 < WD Black = Plextor M8Pe < 850 EVO < Z400s
(Intel is kind enough to provide engineering support, this might change the position of 600p here)
Personally I would go with the SanDisk if the DRAM issue can be resolved, since I stand by the original consensus that faster SSD doesnt give a significant performance benefit on this type of devices, while 1-2 hours of battery life does.
EDIT I was comparing the 240-256 GB models of each SSD since it is the middle ground of the V. Upon further research, the SanDisk is not available in 500 GB variant, and the PCIe options I presented above (ADATA SX8000, Plextor M8Pe, WD Black, Samsung PM961) are quite a lot more expensive than the Intel 600p in the retail market, leaving 600p as a good performance-per-dollar value among 500 GB PCIe SSDs. However, we should not overlook the above-average power consumption that seems to plague the 600p. Unless Intel's optimizatio could mitigate this problem, I would still recommend the 850 EVO over the 600p if it could bring an additional 30-60 minutes of battery life. Also dont forget that Eve might get special bulk discount from either of those companies, so far we only know that Intel cannot provide one. If nobody gives special bulk discount, we technically could even have a poll to have different OEM for each m3, i5, and i7 model. For example, m3 buyers might prioritize battery life over performance, and the other way around for i7 buyers. This makes the Z400s a possibility for the m3 and i5 models.