I think that an upload that doesn't double check the size/presence via a SIZE command prior to uploading probably isn't a good idea if the file didn't exist or the filesize differed from the listing. Using that logic you could argue that this is a "safety feature" so the option "Request file size/date prior to transfer" could just be used to skip the check when the file DID match and you wanted to skip the double check. Alternatively it really could disable the SIZE check all the time like it does now, but then some smarts about when to issue SIZE and/or MDTM could be dependent upon the rules used. Or you could do both ideas.
Hmm... I'm not sure I'd trust just MDTM to verify existence though. It's not universally supported so SIZE might still be needed if MDTM returns a failure. Also, I know I screwed up ioFTPD's implementation of MDTM for a bit because it didn't handle spaces in filenames correctly. It thought you were trying to SET the timestamp so I know I wouldn't trust it's reply all the time
