I'm not sure why people are saying you cannot. If you are unhappy with your device, chances are today is the last of the 14 days you have to return it with no restocking fee, no questions asked.
You can return your iPhone 6s.
You would then need to re-purchase another 6s. A few issues:
1. You aren't guaranteed they will have what you want in stock.
2. You aren't guaranteed the new phone won't have the same crappy Samsung chip.
Theoretically you could continue to return the phone until you got the one you want... but for the difference I'm not sure its worth it to most people.
For most, today might be the last chance though. I doubt Apple will ever compensate people for having less of a phone than others have.. they will find ways around it (as they already stated). I don't know about you, but its a huge disappointment knowing the iPhone I paid for 2 weeks ago is even 2% worse than someone elses, let alone 20% or more. This is something I wouldn't expect from an Apple product.
Now having a "gate" issue after a new iPhone release? This has become an expected yearly happening... always some major issue with the new phones... antenna, bending, and now chip... when will this end, probably never.
Fortunately even with these issues their phones are still superior to anyone elses.