X-FORWARDED-FOR AND ERASING SNAT IP FROM APACACHE LOGS

The XFF header is a layer 7 object, while the SNAT address is a layer 4 value. You cannot use an iRule to change the SNAT address without altering routing and potentially defeating the reason you're using SNAT in the first place.

I would suggest tailoring the Apache log config to ignore the source address, the %h LogFormat value, and perhaps replacing it with the XFF header value \"%{X-Forwarded-For}i\"