SOAP::Lite seems too much work IMHO. Not only documentation is scarce for the alternative, also examples or docs are not clear enough. It will require huge changes that will ask for a complete redesign of the code.
If we move forward with the replacement, I would suggest uploading the files in some other means as we do not know for sure how much memory reduction we can get with
XML::Compile alternatives: i.e.
Tweaking how we deal with attachments I could further reduce the footprint below Ben's numbers (to ~50% instead of ~60%). Clearly not enough but a workaround while we look for replacement.