The Isco II is a fixed mounting too, but I just lift it out of the way for a full 1.85:1 movie since I hate to waste pixels. I use the lens memory in my X35 to slightly adjust the zoom when the lens isn't there and it's a good solution. I also find with the Isco II properly adjusted that it doesn't seem to add any
visible distortion to the image even close to the screen: Any convergence errors that are present without the lens in place are no worse when it is in place and focus is very sharp, these observations are when close to the screen, so sat back in the viewing seat then the image looks extremely sharp and clear.
I'm not sure how much cheaper the alternative is, but I wouldn't want any
additional distortion to the image so if the alternative is cheaper, but worse, then I'm glad I paid more.