similar thoughts and it certainly calls for a good quality flexible grout - not one of the "shed products" and getting it into the joints will be a challenge.
Im using grey UltraTileFix ProGrout Flexible, i haven't any idea how this performs... Somebody did say earlier that 2mm would be fine with good preparation and a solid floor, so now thinking of 2mm as i think 3mm would be too noticeable and i would have to re-plan the whole floor, as i have planned it so i have even cuts, but its 275mm from center of floor and i don't think you would notice because the spacers would look pretty uniform, even with a 2mm spacer... To go 3mm, i would have to start central in the room, so it would look uniform, the only problem would be a small 50mm cut either next to door or next to wall at the other end of room...