Also we have these additional pts to consider :
(1) XTAR 2600s are purchased from XTAR HK as per paypal transaction reference. There is absolutely no way you can get ripped off. Zero percent. They ship fast, i purchased 3X from them and 3X they arrived in my mailbox within 2 weeks, fastest was 5 working days. For the budget brands, you deal with ebay, DX, MF, focalprice. You play with roulette as the source, even the same source may receive fakes. And if you do not test/measure, you will never know. In that case it's better to stick with the small single cell stuff, ie you will never advance past a point, esp if in the future when there are new emitters doing higher currents. We all know that lights are somewhat hindered by cell discharge performance - basically it is 18650 coz that is where all the R&D going to be. Nobody will give you a true 6AH 32600 even though size would dictate it easily could.
It is really the fakes that are floating around, the ones with using reclaimed cells. So many hundreds of reports around and they have been around for years literally on websites/blogs/CPF/flashlightforums etc. This is not a new problem, and with new flashaholics they will always find buyers. Sellers are interested in profit, and many are not technical. Eg, you can't blame DX for selling 22g worth of Fujik when it says 30g on the bottle, but you can warn others and that is pefectly ok/legal.
(2) Quality of PCB. Their PCB protection is pretty aggressive. In my DD XM-L lights, they cut off when fully charged, even in strobe mode. Prob at just over 4 amps. Sensitive as well. It works perfectly ok when charged to 4.1V in my DRY triple XM-Ls...so it's cool (no real capacity from 4.2V till 4.1V, unless you count approx 30-45 seconds runtime as capacity). Low voltage cut also works. This would be a very safe battery to use but still leave enough headroom for high powered lights. I have many other budget type protected cells but they cut way too early like they do not allow me to use in the UF-980L @ 3.5A. Only exception is Trustfire Flames.
I don't know how they protect against overcharging in case your cheap charger goes wonky (most usual case of catastropic venting), but i'll bet they cut perfectly. If a user wants to save a few bucks on cells, i am sure they won't be using hobby chargers or Pila or even XTAR. I already have 2 faulty OEM generic charger, one actually works but cuts at 4.25V and i don't like that. Try some more with cheap cells w/o PCB and play more roulette....
I'd rather run a 4S light complemented with equipment like DMM/charger and Li-ion handling SOP, its relatively safer....
(3) Unprotected. Esp if you are a novice and do not know when and how a driver fails and FETs create a dead short and what are the consequences, you will definitely be facing a pipe bomb case, guaranteed. Even if it's protected. I have a protected cell actually do not protect against shorts, faulty? Again, it's roulette. Obviously if they can fake a cell they can also fake a PCB. I'd not count on it to work 100%, or prolonged usage etc.
Edit - i am not saying that they are bad, but only for other applications with lower current needs.