Proof that you don't need to cut the tile size too much:
As I said earlier, on the current TP build, on my Lumia 920 there's a 21-pixel gap all around the tiled area to the edge of the screen and there's 19-pixel gap between tiles. On a 6-column layout, that means the gaps are 21+19+19+19+19+19+21 = 137 pixels out of 768, which is huge.
I took one of my old screenshots to demonstrate the change in tile size if you maintain exactly the same gutter between the tiled area and the screen edge, but instead cut the inter-tile gap to 5 pixels. I darkened the screenshot to make the new shapes stand out more clearly. Once I'd shrunk the tiles to fit 8 of them across the screen, this is how they look:
As you can see, the difference between the size of the old small square tile and the new one is very small indeed. If I had made the gutter around the edge smaller and/or used a 3- or 4-pixel gap between tiles, the new tiles would have been a touch bigger and thus the difference would be a touch smaller again. I think any phone that is currently displaying a 6-column layout now ought to be able to handle an 8-column one without any problems at all.