The PSU you have is a repackaged seasonic (one of the best PSU makers IMHO) so I see no reason to upgrade it, especially to a lower wattage one. Unless your having issues with it as mentioned above. If it were me I would get a GTX 1080Ti or another GTX 1080 for SLI before I go with a lower wattage PSU (that can't support SLI no less). Regardless you system is very solid and doesn't need much of anything ATM. If you dead set on an upgrade get a i7 4790K and get some more cores and threads to future proof your platform a few more years for gaming.
unless you are having PSU issues, no reason to buy a new one. if you do get a new PSU, a high end 650W is enough for a 1080, but would not be enough for an SLI configuration.
The PSU you have is a repackaged seasonic (one of the best PSU makers IMHO) so I see no reason to upgrade it, especially to a lower wattage one. Unless your having issues with it as mentioned above. If it were me I would get a GTX 1080Ti or another GTX 1080 for SLI before I go with a lower wattage PSU (that can't support SLI no less). Regardless you system is very solid and doesn't need much of anything ATM. If you dead set on an upgrade get a i7 4790K and get some more cores and threads to future proof your platform a few more years for gaming.