i have the ADS box, and do what you want to do. in fact, i got the box just because it takes a freakin' long time to encode mpeg 2 on my G4 450.
you can capture the video to mpeg 2 in realtime using the box and the included pixela software. then, afterwards you can then export the file to elementary streams; it creates an .m2v and an .aif. exporting captured mpeg 2 video to FCP/DVDSP is actually covered in the Instant DVD for Mac manual.