I've used many 3rd party BP-511 batteries in my Canons without any problem. In fact, several of them are significantly better performers than the OEM's that came with the cameras. But none of my Canons cost $5,000..
Now, if you inquire of your dealer you will probably be informed that anything other than a Leica battery will void the warranty. Personally, I think that would be difficult to claim unless a third party battery melted and fused inside the camera. It's the camera circuitry that determines the draw on the battery, not the other way around. I wouldn't be concerned if the battery was from a reputable after market supplier.
I notice that B&H only sells the Leica batteries for both the M8 and Digilux. Nor does my favorite vendor, batterybarn.com, offer after market replacements for Leicas. It may be because the market is so small, compared to other digital cameras, that no replacement manufacturer has seen fit to produce them.
I will feel confident using an after market battery when it is sold by a reputable vendor like BatteryBarn or B&H and comes from a named source such as Monster Power. Unnamed from an auction page? Probably not.