The Sigma 150-600 Sport is over 2000€, you can't buy it and a D7200 for the price of the Pentax - unless you can tell me where I can find D7200 for 200€ (I'll buy two!). So it has to be the Contemporary version, with different optics and construction.
AFAIK, the Sigma is losing performance at longer focal lengths. The only thing I can use to compare the two lenses is ephotozine.com, who tested both lenses - the Sigma on a Canon 5DMkIII, which I think it should give it an advantage.
I know it's too little, and I reserve further judgement when we'll have more data (and, perhaps, hands-on experience with the Pentax).
By the way, I'm not saying the Sigma is a bad lens - it definitely has its place as an "affordable" long telezoom. Just inferior to the Pentax
Jimmy, perhaps I could explain myself better in our native language and with some cold beer on the table.
Please understand, I'm also ignoring my own wishful thinking and try to be realistic - with the risk of sounding cynical and unsympathetic. The sad reality is that Pentax cannot price their high-end products with us in mind - so we have to do what we can, i.e. getting the best equipment we can afford.
And I'm sure a D FA 150-450 made to sell for half the price (MSRP) would be a different product, weaker than this one. Some sort of a 'Contemporary' version, perhaps not even that - due to Pentax' lower volumes.