With all due respect, if you can afford an M8 or an M9 I cannot imagine why you would try to save a few bucks or euro's by not buying the original.
I don't think it's worth the risk
If money always bought you quality in proportion to the tag?
I suppose I should upgrade my garage to a Cadillac garage, and drive the car only on new highways, and then there are those Cadillac wiper blades, oil, and batteries. I gave up trying to find Frank Lloyd Wright paint for my house and bought Sears, (made by Sherwin Williams).
There was a time I only used Kodak printing paper and chemistry.
Perhaps such a priced camera should come with a spare battery as well, I suppose I should be happy they come with a strap.
I have been told by folks who work in camera retail that the mark up on accessory items comes close to 8x, and I have experience with branded camera batteries from other sources which were not only over priced, but the quality was just not there as well, nor the warranty. Biggest local shop sold a spare battery for a $150 Casio for $50 -- which died in two months, and their response was that happens with camera batteries, but they would order another for another $50.
I still do not know why it is so difficult to make and find good batteries for just about anything, as far as I know, Leica does not make batteries, and no one seems to know who makes them?
I suppose I got lucky with a couple of spare batteries (fingers now crossed) for mine, they have performed pretty close to factory, so far, but I have only had them for several years.
Still, if I see some Leica branded batteries at a reduced price-- and have any spare change after sending Leica a few pay checks-- I will consider them.
I do use Swiss watch batteries, the cost $2 more, not 10x, there are times when you get what you pay for, trouble is, determining that in advance.
If I only pony up $1000 for a used M8, then I will feel OK using unbranded batteries.
Regards, John