Which version of Java?
Which web app server/db and version?
Does it work for small (<10Meg files)?
I've never needed a timeout over 4 minutes even for 500Meg+ PS files, although I'm interacting through the EJB interface, not the webservice.
I believe there may be some way to increase the timeout further, although I think it involves editing configuration files within the ear/war/jar structure of the deployed libraries; the 270 second value does appear in native2pdfconfig.xml in the pdfg-all.ear on the professional version of PDFG, although I don't have time to test whether this value actually has an effect on the maximum timeout. It may appear elsewhere too. And I'm not sure if it will be in the same location in the postscript only version.