I bet it would be about 2k like it is to upgrade to the 2.0T Ecoboost on the Escape. So a GT with 2.5T just under 34k (just under 36k with PP). That's still competitive against the Escape (though they discount those a lot more). Also, still competitive against the 2018 Tiguan when similarly equipped and I'd argue even though it's got more torque the VW 2.0T Bcycle is actually more comparable to the Skyactiv 2.5. Mazda would sell them.
I also doubt the diesel would be as quick as the 2.5T. Yes, they have the same torque.... but that's just one number. I bet it would drive a lot like a VW TDI. Basically not that quick but no matter how much stuff or how many people you have in the car it'll accelerate the same.
Definitely not slow. My point is that 0-60 (100kph) should be about 2 seconds faster with the 2.5T I bet. So it's a different customer who would want the diesel vs the 2.5T in my humble opinion. And both customers exist in large numbers so I think Mazda would sell plenty of both to make it worth their trouble.
The 2.5T will be faster in sustained straight line acceleration, 0-60, 1/4 mile, etc. simply because of the higher horsepower, but I doubt the difference would be as large as 2s from 0-60. In normal daily driving they will feel very similar. When I drove the 2017 CX-9, it had the most diesel-like powerband of any gasoline engine I've tried. The 2.2D will have more grunt just off idle which you'll feel when casually rolling off from a stop, and the 2.5T will rev out an extra 1000 rpm in the lower gears when you have the pedal mashed. But with either engine you'll spend the vast majority of your time between 1500 and 4500 rpm where they are very close to the same.
I don't necessarily agree that they appeal to different customers. Obviously, there are some people who won't buy a diesel for no other reason than it's a diesel, but who would buy the 2.5T. And there are a few people who are mainly interested in the diesel for fuel economy and who wouldn't buy the 2.5T. But I think there's a lot of overlap between the potential market for the two engines. I'll bet that if we had the chance to drive both of them back to back, most of us could be happy with either choice.
If you want to know what the 2.5T is like, test drive the CX-9. It's not anything like the 2.0L turbos in the Forester and Escape, which are tuned more for high rpm boost and when you mash the pedal you feel the lag followed by the turbo push as the revs build. The Mazda 2.5T is tuned more like the VW TSIs, with minimal lag and plenty of boost at cruising RPMs, but running out of steam as you approach redline.