Can you tell us more about these IR/UV filters as I'm sure I'm not alone in never having encountered them before. What do you mean by different strengths? How much are they? Are they widely available in different sizes? Is there a filter factor? Do they have any other negatives? And finally, is there any way you could post shots showing the practical effects of filtration?
Without going into great detail and sound like I'm lecturing I will summarize and skip technical accuracy for more easily understood plain English
There are two styles of IR cut filters, "hot-mirror" or "aperture grille" and "absorption". The hot-mirror actually have a metal oxide deposited on them that has slits that allows visible light to pass through but prevents the larger IR rays from passing. (Kind of like the door to a microwave oven, only for blocking light and not high energy radio waves.) The metal oxide gives them a mirror finish on the outside, hence the name. By varying the diameter of the slits in the oxide coating, any exact cut-off point of IR band can be implemented. Usually, this will be somewhere between 700 nanometers (blocks IR in tungsten light) and 770 nanometers (blocks the IR present in visible light).
The absorption units are colored a light cyan, but attenuate IR by altering thickness and density of color, not just density. And the final thickness is critical in determining the exact cut point, so manufacturing is more difficult than the hot mirror -- thus these absorption/attenuation type filters are about 2x the cost of the mirror units.
The problem with the hot mirror is that the slits effectively get narrower as light begins to enter the filter from the side -- basic trig. This in turn can cause a visible color shift as the slits narrow to the point where they start to cut visible light rays. (Now you know why certain medium format digital backs have issues with color shifts across the frame when using wideangle or shift lenses -- they have hot-mirror cut filters over the sensor instead of absorption type )
This problem is exacerbated for Leica in the M8 since the lens sits so much closer to the sensor than a comparable DSLR. Because of this, even normal lenses have too extreme an entry angle to use a hot mirror filter, so the absorption style must be used. However, the thicker the absorption style, the more refraction you will have at the image corners, and refraction directly over the sensor will impart a form of CA that is not lens related and is not easy to correct in post... So for Leica this was a design nightmare, basically having to walk the razor's edge and balance no CA and a bit of IR leaking over blocking all the IR and having CA in the corners of images.
In the end, it appears Leica chose the 770nm cut point as it deals effectively with IR present in visible light. However, I am of the opinion Leica will have to offer buyers this choice and offer to install the thicker 700nm cut filter as a sensor option -- photgrapher's, pick your poison...
To put price in perspective, a 3" x3" square absorption filter costs around $300. Smaller circular sizes are not much cheaper as they need to be cut and there is wastage. By contrast, circular hot mirror filters will be in the $100 range depending on size and manufacturer.
I'll try to post some example images later this morning.