The exim.conf file works from top to bottom. So a quick perusal of the file will tell you if mail gets forwarded before or after a SpamAssassin check. (I'm not looking at the file now, but my guess is after.) I don't know how safe it is to move it.
I'm eventually going to move to SpamAssassin checking at data time (in the smtp dialog). So any changes before then, and you're on your own .