Just a few things about how we will use this that might help define what is done.
1 - I've just talked to our other computing service who look after the main OU Student Records system and will receive these exported files. They can adjust their code to cope with the trailing ' %' in scores and so it will be OK to use the normal percentage format for export.
2 - Irrespective of Grader report settings we will always want to export a percentage. So even if our Course Staff have set the Grader report to letter, for example if the course is only pass or fail, we would still want the XML export to export percentages. So the export should not be tied to other Grader report settings.
3 - And to stress that we only want to export percentages as integers; rounding up should be done before export. In part this is to ensure that Moodle controls exactly what appears in our external Student Records system.
I think we can add gradestyle and decimals selectors into grade export form.
Would it solve the problem?