Hi, I actually came across your post on google, as I'm a member here but looking for similar info for my 940.
The original orifice valve is fixed. There is a variable one that some have been promoting as more efficient, not only for cooling but for longer life of compressor and other components, as well as better mpg. It further appears that vov's may all be made by Nartron Corp in MI. I am going to contact them for more info.
It appears that the VOV, Variable Orifice Valve also known as an AAOT, Auto Adjusting Orifice Tube, is made in two versions, one for 'normal' temps of under 105, and one for severe temps of over 105 degrees. I'm near SF so the 'normal' is the way I'm going.
Many have never heard of a VOV. The AAOT term seems more widely used.
I have found that prices vary, and one place,
http://www.acsource.com/smartvov-highperformanceorificetubes.aspx
seems to have it 'on sale' for $10 for the 'normal' application. Other places list in the $20-$27 range, and a few are as high as $45. If there is only one maker as I am thinking, well draw your own conclusions.

All the above as I've said, is 'appears'. I don't say that all of it is gospel yet, but it does seem this way so far.