The publish to PDF step is worth a try, especially if you have a more recent version of CorelDRAW. The PDF export engine in version X3 is the best I've seen from the application (although one of my coworkers can't seem to e-mail some password protected PDFs generated from CorelX3...wierd).
If that doesn't work, I would definitely consider the Photoshop alternative.
Also, are you sure you need 300ppi resolution? With the artwork at full size, I would only use 300ppi levels for print jobs where the viewer will be standing close to the display. There's lots of jobs where I compose at 72ppi, such as a 3' X 8' bus bench ad or a large banner. I'll do a 10' X 30' billboard at 36ppi or 72ppi, depending on the job complexity. A 14' X 48' or larger board can be composed at 36ppi or even as low as 18ppi. That resolution may seem pretty coarse, but a motorist on the interstate won't notice the difference.