Here is what I would do: In System Preferences>Print and Fax (it might be Print and Scan now—I am writing this on my 10.6 computer) remove the StylusPro7800 and Quad7800 from the list of installed printers in the left hand side of the window.
Re-add the StylusPro7800 and choose whatever the latest Epson driver is in the Print Using drop down
Then, still in print and fax, add the 7800 again, and this time choose QuadToneRip version 2.7.5 in the "Print Using" drop down option. That should load the printer as Quad7800 with all the paper sizes and allow you to print using your installed your profiles. You might still need to run the install script in applications/QuadToneRip/profiles/4800-7800-9800-UC.
Hope that helps,
Richard Boutwell
www.BWmastery.com
www.RichardBoutwell.com