Power plants are the same as the 139's, so there's only so much you can squeeze out of performance.
Put more weight on those donkies and you are going to end up with just a bigger and heavier underpowered 155.
Let's not forget that the 139 is certified for 15 passengers away from the land of "Supersize Me".
I don't think the 175 will be underpowered, I hope they are going to start from 7.000MGW like the 139 will be this summer or later, there is power to add a couple of pax or more fuel, as some people have said in this forum we can reduce the OEI performances for a few seconds.
The 15 pax. option in the 139 isn't very comfortable, so it's really nice a bigger cabin, if they do it right the 175 I'm sure will be another winner.
Regards
Aser