Currently have as KACM4 RAS and and 6P/VTAC light mount combo. I'm likely going to dump the quad rail and go to the Magpul SL handgaurd.  Looking for a mount that can get the tailcap of the light at least even to the FSB and be as low profile and light weight as possible. Any reccomendations?

I have an Arson Machine mount on my wife's AR with the SL handguards.  But the tail cap does not get to the FSB, I think, it's been a while since I have pulled it out of lock up.  Assuming you are referring to the forward placement of the light.  I know IWC makes some that cantilever out there further, but you will have to kind of dig through their site as their site kind of sucks and they have like 3-4 different models for the MOE handguards.  But really those are the only 2 options for the MOE type handguards IMO.  The plastic mount from Magpul is just terrible.  I don't think Arisaka makes one that will work with the SL or the MOE guards.  And everything else out there is just not quite as nice as the aforementioned mounts

I knew about the IWC, but wasn't aware that mount worked with the new SL models. Good to know.  Thats the route I'll likely go, the tailcap gets slightly aft of the FSB which is about the best I'm gonna get from a carbine handgaurd.

Arisaka Defense MLOK mount

I have the IWC XL mount on a SL handguard. I really like it because it extend the light forward a little more. I really really like it. It's not the sexiest mount, but it worked just like I needed it too.