Will use of FireVault reduce lifetime of SSD?
I can't see file vault impacting the life of the drive since it's a block level encryption and doesn't add or subtract any overhead in terms of what data is written.

Put another way, if you asked - "Does writing 80% zeroes to a drive reduce the life of the drive over writing 80% ones?" the answer would also be no. The life of the drive is based on power on hours and write volume and not the exact values of the data being written.