I have a 2011 WK2 Hemi, fully loaded. It is essentially the same engine, I just think that the gear boxes are different in the 2014 WK2 and some styling. I think for an SUV of it's weight class, it's pretty fast. Do note though (I can't confirm this, it's an assumption and I'm too lazy to look it up) but the Hemi JGC is a lot heavier than the X6. Even if it isn't it's not really meant to be a sports SUV. If you want something that encroaches on luxury SUV territory, looks aggressive, and can offroad pretty damn well (probably one of the best stock SUVs to buy for offroading bar a few) then the JGC is your weapon of choice.
I personally have one major complaint and it's that here in Kuwait, the service center is awful. I'm the type of buyer that likes to keep a vehicle for at least 7-8 years minimum before I even think of buying another vehicle let alone actually buy one (that's usually when I start saving and budgeting). My last SUV was a 98 Pathfinder, bought brand new. Sold at the end of 2011 when I bought my JGC.
As for the power, I have heard of power upgrades but haven't really gotten into them. It's not a vehicle I think should be fast and even the reported 7 second to 62 from a stop is not that bad of a time. I can report that I've done my 0-62 (if I remember correctly) at 6.7 seconds and I live in Kuwait (I could be a tenth of a second faster or shorter, I did it fairly early on in the life of my vehicle and I'm at 41,000km now).
However, my service center is so awful, I'm already thinking of getting rid of my JGC. Right now it's looking like I'm going to either do one of the following:
1) Trade it in for another SUV (since I spend my weekends in the deserts and off road, not major offroading but not just any SUV will cut it).
2) Keep it, break my rule of always servicing and taking care of it in the service center and do it outside. I'm currently thinking of picking up in October when it arrives in Kuwait an MK7 Golf GTI. VW/Porsche and Lexus/Toyota are reknown for being the best service centers in Kuwait. Price, quality and everything.
So I'm not too sure yet. I'm going to look at my financial standing come October but I've put my name down for a black GTI (in case I decide to). They're going to notify me once the vehicles come in and they finalize the pricing.
I'm really upset because I really do love my hemi JGC. It's an absolute joy to drive and no matter what car is next to me it is an attention grabber. I got mine in mineral gray. All my clients love it (I have a small design firm, we focus on application development but we work in all sorts of designs from print to digital to billboards to rebranding etc. I do a lot of technical work and business development so I'm always with clients, they love my JGC so much they always want to ride with me. Note, two of the clients that love my hemi baby drive a Lexus IS300 and the other a Toyota Land Cruise V8. They still think my Jeep is a lot more comfortable and the gent with the land cruise wants to sell his for a Jeep GC).
All in all, if you're really into power, I'd advise against it but it's no way a complete slow poke. It's just not as fast as it looks. Which I think is a good thing. Most SUVs in this range I think should not be that fast. All it takes is one nasty pothole at a high speed and you can easily flip it. I've seen it happen to my friends X5.
It's got almost all the luxury that you'd find in a vehicle twice it's price, and is extremely comfortable to drive in. In almost any mode. I'm usually in "normal" when on the highway, and I drop it into sport occasionally on the highway but to be honest, I don't notice a night and day difference between the two. There is, but it's just not going to turn your SUV into a speed machine.
Just my two cents.