I had a 2012 690 enduro that came with a Leo Vince slip on.
I found it loud so I re-packed it. Not much better.
With the Leo Vince the bike would stall almost immediately. Would restart and run fine, but occasional stalls would happen at stop signs or in opportune times on the trail with the clutch in.
The fit wasn't great and it took some fiddling. Also I didn't find the quality of the Leo Vince to be particularly good.
I found a stock silencer on Ebay and put it on. Obviously quieter, no more stalling or running issues and no impact on power delivery, and the more reliable running characteristics made it much more enjoyable off road. Also, to be honest I found the increased noise annoying, both while droning along the road and while off road.
The ONLY upside, is weight reduction and even then it's pretty inconsequential. It's a big bike, and the couple/few pounds knocked off won't make it a single track weapon.
Since you asked, I wouldn't waste my money on a slip-on A better seat, perhaps bar risers and a gearing change if you want better tractability off road.