the SID is the lightest fork out there to my knowledge. manitou's mars series and sx ti were good, lightweight forks as well. anything else, though, and you are at the ~3.5lb range. to get worthwhile weight reduction you have to go SID or manitou mars or total-air a Judy or something.

fox and white bros make good forks but definately will not give you the weight reduction you're looking for. even the skarebs will not give you much reduction from what you currently have.

ditch the discs and get a 1500g wheelset and you will feel the weight difference. for sure.