In regards to the lower kV for extended life, I'd need to ask an x-ray tech to understand completely, but my understanding is that the applied voltage (kV) or the current (in mA) is varied according to the performance characteristic needed (this is automatically set during the product calibration cycle). Multipling the kV and mA gives you the wattage (ie: 50kV @ 5mA = 250W) which is the tube's rating. An x-ray tech would be needed to explain which (voltage or current) is what would de-rate the tube's life most quickly. I would expect if either were too high and approached the tube's maximum rating in watts, it would be a bad thing. I think these machines are all designed to stay well under the tube's rating when they are new, because as they age, the kV required to do the same job increases, until you are max'ed out.
Consequently, the advertisement that they only run at 2/3 power should be taken with a grain of salt because it means they have 1/3 reserve and they need that reserve as the tube grows old. Beware that the kV settings for similar products can be quite different. I've seen defaults of 30kV to 50kV on my products, and the ability to manually adjust these levels can be very useful, however that wide 20kV range means that I'll consume my 'age' safety factor more quickly.
Another parameter I'm looking at is the diode pitch used by the detector. This essentially defines the machine's measurement resolution, and high resolution means you can spot smaller contaminents. I believe my current machine has a 1mm pitch. The industry standard might be 0.8mm and I've been told that a few companies manufacture machines with a 0.4mm pitch (Anritsu and Ashida for certain). I'm now looking into a model which uses a 0.2mm pitch. The software imaging with multiple algorithms are probably the most powerful feature of these machines (separating the good machines from the very good), but like a computer, the hardware platform (or hardware resolution) is the basis the software can build on, so a small diode pitch would seem to be a very good thing.
Regarding usage as a checkweigher, even with a non-homogeneous product you might have some application for the mass measurement function. Use it to double-check your checkweigher (using a seperate output to alarm the user) or based on the known measurement resolution per product, blow the product off at the resolution limit, or use it to find missing products in multi-product packages. As long as the computer platform has the power to do this in parallel with the x-ray scan, at your product rate without any bad effects, why not use it? Where most of the processing is done varies by machine, but a lot of the application and user IO is the computer platform, so I'm looking at machines with modern computers running dual core processors. Touchscreens can be annoying and overloaded computers make them even worst! jmo