Yes, but if you use the resolution you suggest (1704 x 960 @ 5.5"), than you are simply stretching the image. So, basically, everything you see on the 4-inch iPhone 5S right now, you'll also see on the 5.5" iPhone 6 - except much bigger. Much bigger. 1.4 times bigger, in fact. Text, images, apps, icons, etc. - everything will be 1.4 times bigger with the resolution you propose.
Apple is not doing that.
For you to compare: the Safari icon (or any app icon) on the iPad Air is 1.24 times bigger than on the iPhone 5S.
So, here's an explanation for the resolutions used in the 4.7" and 5.5" iPhones (assuming the suggested resolutons by Gruber are correct).
4.7" iPhone 6
iOS points resolution: 667 x 325
Resolution @2X Retina (as rumoured): 1334 x 750
PPI @2X Retina: 326 ppi
What happens here? Everything - text, icons, images, etc. - will remain as big on the 4.7" iPhone as they are on the iPhone 5S.
What has Apple done? They have added 99 points in height and 55 points in length (5445 new points).
The result? The display will remain as sharp as on the iPhone 5S and everything remains the same size, but now way more content can be shown (5445 extra points for extra content - or 1.4X more space for extra content).
5.5" iPhone 6
iOS points resolution: 736 x 414
Resolution @2X Retina: 1472 x 828
Resolution @3X Retina (as rumoured): 2208 x 1242
PPI @2X Retina: 307 ppi
PPI @3X Retina: 414 ppi
What happens here? Unlike with the 4.7" iPhone, everything will become slightly bigger. How much bigger? 1.04x bigger. Basically, sizes of app icons, texts, etc. will be bigger than on the iPad mini but smaller than on the iPad Air.
Additionally, just like on the 4.7" iPhone, extra pixels in every direction have been added. How many more? 168 points in height and 94 points in width (15792 points for extra content).
The result? Everything will look ever so slightly bigger (1.04x) - one of the things customer expect of big screen displays. Additionally, much more content can be shown (15792 extra points for content - or 1.7X more space for extra content).
There is only one problem: everything will look slightly fuzzier, because at 2X Retina, the pixel density will be 307 ppi (19 ppi less than before). And that's where the 2208 x 1242 resolution comes from - because that delivers a "3X Retina quality" - which boosts the pixel density to 414 ppi and makes it easy for developers to adjust artwork.