If you're going to make a statement like that, please refer to the source. Which experts are you actually talking about?
My opinion about Sauber differs from some of the people here. Although ive lost all interest and faith in the team since 2013, and have little good to say about their current team principle, I actually think they will improve in 2017. They have struggled financially for a long time, but once they got the resources in place, they showed decent progress in the final part of this year with a few crucial updates. Also they have been hiring a lot of great people recently. Focusing on their main weakness, aero & chassis, makes a lot of sense to me. Having a fixed PU specification will be beneficial throughout the development process of the rest of the car. Yes they will ultimately lose out on some peak power, but I expect them to overcome that disadvantage with greater chassis improvements. Regarding the free-for-all PU development which will be allowed next year, I dont see that making a massive difference. The leading PU manufacturers are already into the zone of diminishing returns, with ever decreasing improvement rate. I think the year-on-year performance difference will be slightly less than it was from 2015 to 2016.
I expect Sauber to quite easily beat Manor through the use of better structural resources and possible financial backing. If they were to sign Wehrlein, they will probably have a better driver line-up than Manor too. I expect Haas to struggle next year, having not had unlimited wind tunnel time this year, so Sauber might beat them too, at least at the start of the season.