I'll start with the grand finale - the HK P9SK shoots amazingly straight and very comfortably!

From the first mag I was astounded at how accurately I was able to shoot it. My friend even looked over at a group and asked "How many shots was that, 3?" I replied "Nope, 5, it shoots that straight." He replied: "Let me try that thing." And he too was pleasantly surprised with the grouping.
I think I shoot it better than my Glock 43! It definitely is a lot more pleasant to shoot.

Of course, if you look at the dimensions, you see why. It is thicker, not only in the grip due to its double-stack frame, but also the slide. The VP9SK also weighs about 5oz heavier - unloaded. I didn't take it apart to see if there was anything special in it, but it definitely shoots more pleasantly than say an M&P Compact. The trigger is better than both the Glock 43 and M&P 9c. 

There a coupe of things that I should point out.
First, the slide lock is low and long. I found myself accidentally hitting it with my thumb, causing the slide not to lock back a couple of times. This is a training/practice issue. I personally prefer them to be small and out of the way, but I know that others do not.
Second, the mag release it not traditional "U.S. Style" meaning that it is located on the side and activated by your strong thumb. I actually don't mind this as prefer them to be out of the way also, but it is definitely not what I am used to and unlike all of my other pistols, except the Walther.

So, in summary, I do not think that the VP9SK is in the same deep concealment class as say the Glock 43. It is also considerably more expensive. The 43 with MagnaPorting is still less.
However, I think that accuracy is paramount and HK has a winner. Its shootability and accuracy out of the box in this size/weight class is something of an anomaly and I want one. HK needs to come up with an instructor program! 

Glock 43 vs HK VP9SK

Glock 43 with Taran +1 baseplate vs HK P9SK. The HK is still noticeably longer and wider.