OK, well its not opinion what I stated, but fact about the common poor quality and false mAh in “Ultrafires” in the general markets. You MAY have gotten lucky and gotten an Ultrafire with OK capacity, maybe you got a “real Ultrafire”. You could have also gotten lucky by getting a recycled used 3100mAh Panasonic that was re-wrapped as an “Ultrafire”. Who knows really…including yourself! You also noted you weren’t certain the other was a Sanyo cell, and you have no way of measuring capacity…so really you have nothing to judge it from, and if both batteries are low capacity they both seem “fine” and your Z5 runs the LED on high at 1A say (most are far lower current and lumen output than you think, here a Z5 is measured at 1A tailcap current on this site: Review: TrustFire Z1 XML-T6 "1600 Lumen" Zoomable (a.k.a. Z5) ) for a relatively “long time” of say 2hrs, so from your perspective they seem “good”, but still have only 1,000 mAh capacity each… The thing is, if you have no “known” quantities, you cannot solve the formula. All I’m doing is warning you to be careful and you can do what you want. 
An easy way to compare would be to buy two Panasonic cells of 3100 mAh from a reputable dealer, and compare run time to when it dims. Or with a cheap multimeter, take a tailcap measurement for how much current the led pulls, and run the light, monitoring voltage in the batteries every so often as it runs down in a timed manner, and you could get an approximation. You should never go below 2.5V, many batteries 2.7V, but you really dont know what you have so it would be hard to advise you…but best would be stop once you hit 3.0V, there is very little energy after that anyways, and batteries will always jump back up in voltage once you take the load off and take them out of the flashlight, so I really wouldnt run them further than that if you do this test, thats a pretty good approximation. And of course they could still blow up doing any of these tests, so do any of these tests at your own risk knowing Ultrafire cells are garbage and could explode and kill you! Probably wont, but its like a negative lottery, if you are willing to roll the dice knowing it could happen, its up to you, I wouldn’t. I’d buy some good Panasonics…in fact I did exactly that.
My first experience was with a Z5 (and a couple other lights bought for gifts) and crappy Ultrafire batteries. I found they drained quickly, and I started to do some runtimes on my batteries I purchased, and bought some known good Panasonics after finding out how bad the Ultrafires really were when I did some research. I had purchased several sets of red “ultrafire 3000mAh” and on comparing them with known good batteries I found that these had about 1/3rd the runtime on average…and as I used them a few times the capacity/runtime went DOWN in those. One other “bargain” group of “5000mAh” batteries I bought had 1/10th the claimed capacity. Luckily I was able to return many of the batteries and some sellers didnt want them back, so I recycled those, I have none left. I won’t touch Ultrafire/cheap batteries anymore, only Panasonics, Sonys, Samsungs, etc. Just isn’t worth it for the risk, run time or low max A output.