I gotta say, I switched the bar on my ms660 to one of those Stihl light bars and it made a world of difference. It took chunking down a spar from agony to bearable. 36” bar too.
Ms400C is a tough, rippin’ saw. Mine is still killin’ it. If you want something lighter, but still has enough power, I’ve heard great things about the MS261. It’s next on my list. That and the 500i.