You got me wondering just what I had done, so I made a 300 dpi CMYK + Spot image in Photoshop and saved it as a DCS 2.0 (5 plates plus master) image. Then I imported the master eps file into Quark 4.11, and printed both seps and a composite to my HP Laserjet and as 2 composites to my Epson 1520 (at 720 and 1440 ppi, via Adobe PressReady).
Results: Seps turned out as hi-res as the 1200 ppi laser would allow. The LaserJet did show a 72 dpi bitmapped composite. The Epson prints, using "Printer" screening, were equally pixellated.
I made a new document with the Beta of Quark version 5 and sent a composite to the Epson. Even though the print box is a little more complex (which led me to hope), results were similar to those from Quark 4.11.
So I guess I would have to say you were right; the composite is not as smooth (though it would have been quite useable for presentation purposes, as long as you don't need small type rendering). If you need the smoothness, then possibly you might need to create a composite EPS (maybe 150 dpi with spot colors converted?) and swap in the DCS for the prepress output.