My opinion based on my limited experience:
1) is this type good for sling work
There are a lot better. But if you want a people mover that can lift, rather than a lifter that needs to move people, it's worth consideration.
2) what speed and f/f do you normal see near gross wt
Depends what you are lifting but 80 knots is ball park figure. There's a lot of aerodynamic drag in the wide cabin to add to the load drag, increasingly so as the nose goes down. AS350 probably better.
3) how comfy are the seats after a couple of hours
Better than any Bell seat I've sat in.
4) does it handle similar to a AS350 b2
No experience on B2 but probably not
5) any rumors of external gross wt increase
I don't see this as a practical necessity. Gross MAUW external is 2800 kg, hook rated @ 1160kg. All the figures come together at low fuel. You can lift the max but you don't have a lot of power reserve to play with.
If you remove seats to add performance then c of g will be on aft limit, unless you're a big 'un. You will run out of forward cyclic. Ballast in nose helps but I felt that the machine seemed to be unstable fore and aft compared to lateral.
LH seating is okay but nose of the skid always seemed to be in the way of the sight picture.
It's a long scan from the load to the VEMD when things are getting tricky.