You may want to consider reading up on SPLITANDMERGE http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=... which is designed to address large reports.  

"The PDF output for a very large report may exceed the memory restrictions of the FOP rendering engine. In this case, you can split the report into several smaller sections. Each section is written to disk as a separate temporary file, and merged into a single PDF file once the entire report has been processed. "

Looking at your xml structure you may be able to define the repeating element to be <GrItems>.  Note while I do not believe you are including page numbers in your ReportDisplay, but if you do I do believe it does not produce the desired result when using SPLITANDMERGE.