The down travel is limmited by the UAC hitting the spring.
The main problem with strut spaces is that because you have spaced the shock, down, the bump stop doesnt do its job of limiting up trave, to prevent damage to the shock. The shock then free to fully compress as it has been spaced down away from the bump stop, and nothing limits up travel, becauseThis constant compression of the shock over time leads to its failure. Espacially with poor quality shocks such as the factory ones
If you space down the bump stops accordingly, in theory you should be fine. Personally, i wouldnt run them, but my mate does. He has a 2 inch Old man emu lift with a spacer to give him another 2 inches.
He has had this combo for 2+ years and has never had a problem. He only spaced his bumpstops down 6 months ago. (After seeing me break a bilstein shock because of bumpstop issues) This may be because the OME shocks are better able to handle the abuse compared to stock ones. I have seen tough dogs break through the use of a spacer though.
viewtopic.php?f=8&t=9377&start=20 Consequences of having no bump stops, very simmilar to using a Strut spacer without spacing down your Bumpstops accordingly