Instead of running the narrower subway all the way to the floor, have you considered a wider ("taller") tile for the first course, to simply act as a baseboard or transitional piece from floor-to-field? Doesn't have to be an actual baseboard type of tile, or even a contrasting tile. The tile can be the same color as the subway should you prefer it to blend. It can be a simply 6" or 8" square field tile from the same manufacturer. As you know, blending colors may help with hiding a discrepancy, contrasting colors can accentuate it. On the flip side, you could separate the base from the field with a liner or some other detail that melds with details you have higher on the wall, at the chair rail for instance. Again, if the liner contrasts, it may accentuate. A basic 6" square tile, or to keep the subway shape, a 6" tall rectangle, would absorb the floor slope better than a "shorter" tile like your current subways. A 6" tile going from 5" to 6" would hide the discrepancy better than a small subway field tile going from 2" to 3". An 8" tile from 7" to 8" might be even better. If you were to use a true base cove tile like a sanitary cove base, use a basic sanitary instead of a shouldered sanitary, as the shoulder will be cut off due to the slope and that could create lippage or padding out issues as you go down the wall. Sometimes I've even used a taller painted wood baseboard to transition from a sloped floor to the field tile on the wall. An 8" tall wood baseboard scribed from say, 7" to 8" to hide the floor slope, with cap and base detailing, would pretty much hide any height discrepancy from the slope/scribing. The easiest and least expensive "all tile" solution would probably be a basic 6" square tile, scribed and cut from 6" to 5" as needed.

Others with vastly more knowledge than me will pipe up here I am sure as to what could have happened but wanted to relay my experience with large tile. A few years ago, we had 20x20 rectified edge porceline (travertine look-a-like) tile installed over a good sized area (about 1000 sf give or take). I wanted it laid on a diagonal, which called for more tile than laying it strait. We were installing with 1/8" grout lines. When the installer arrived, one look at the area to be tiled, and he recommended to me to lay it strait, as there were many angles in the layout, and I would loose the diagonal effect with an excessive amounts of cuts with such big tile. He went on further to explain upfront, that with such big tile laying on slab (previously wood)that there would be some higher tiles than others due to the levelness (or unlevelness)of the floor. It's just one of those things with large tile, but he wanted me to know up front, so that I did not think he did it wrong or sloppy. He did his best to keep them level on the edges but I do have a few places. It is not noticable to the eye (perhaps floor vs wall) but when I go to push in a chair at the table, I hit a "higher" edge. Nothing to stub one's toe on, but just those places where it's "higher". You probably see a great difference now without grout. I would bet once grouted, the difference will be waaaaay better. However, that is the nature of using large tile.

Thanks for the quick response, Bill! I really trust my contractor because he's done a really excellent job with everything. So... I would guess its an acceptable drop, just not something I'm used to having. I certainly don't want to start grinding my stone down!! Thanks again for the quick answer. I'll talk to him again tomorrow and we'll decide if the level is O.K. He's always willing to go the extra mile to make me happy!

Grout sealer will make the problem worse normally. A grout stain will probably work. I would tell you the stain that I would recommend but we get banned from forum for helping people. This color difference can be one of two things..maybe three. Too much water during wiping....inconsistent water mixture...different dye lots of grout. There is an excellant stain out their that will work...it can even be color matched to the grout you expect it to be...