Hi Paul,
Glad you got it solved by recreating the report. The reason I thought this would solve the issue is because I've seen similar strange behaviors a couple of times in the past.
I don't have a good explanation for the behavior (I'm not part of the SAP team), but I suspect it is related to handling of custom paper sizes and may depend on whether or not the old printer is still defined on that machine. Please let me know if the old printer is still defined on that machine (even if the physical printer was removed, the printer definition or driver may still be available on that machine). If it is, please test to see if printing performance to the new printer (using the old report) is improved
a) after removing the old printer definition, or
b) after removing the printer driver?
Perhaps this would provide the evidence we need to direct SAP's development team attention.