Let's start with the fact that the product page originally said 1500 mAh, but now claims 1700 mAh. Go ahead, click the link from the original post and see for yourself. Hmm, I think they might be jobbing you--what say you? To me, that's proof enough, but let's continue anyway just for fun...
Next, I should point out that they are clearly counterfeit based on the photos. If they are willing to put an HTC logo on a battery that is obviously not made by HTC, what's stopping them from claiming a couple hundred extra mAh too? You guessed it--absolutely nothing.
Now, let's move onto the pure economics of it. If HTC, with its vast resources and R&D, can't manage to cram more than 1400 mAh into this form factor, what makes you think a cheap knockoff factory has figured it out? Well I'm here to tell you that they haven't. If they had, HTC would have bought their technology up before they even had time to look up the definition of
vertical integration.
Lastly, let's look at the competition. Seidio and Mugen, which are reputable companies with heaps of name recognition,
have been known to
overstate the capacity of their
batteries. Point being, if companies with something to lose can't be trusted to accurately state their batteries' capacities, what makes you think that a nameless, faceless, knockoff shop has any incentive to do so?
So in response to your first question, as much as you are willing to lose.
Edit: Attached screen shot of Amazon product listing just in case it gets changed again (to 1800mAh, perhaps?). Note the 1500mAh tag in the address bar, but the recently changed 1700mAh in the product description.