BeBop,
Yes the sink tips provided with the GrandSpey with tips will work very well with the same size MidSpey, or vice-a-versa. If you already have a MidSpey or Windcutter in the same size, simply buy the GrandSpey without tips, cut it 15 to 16 feet from the tip, put a loop on both ends, and then use the sink tips that came with the equivalent MidSpey or Windcutter.
Unlike Fred, I have no problem with cutting a line and using tips from floating to Deep Water Express on the line. To be fair, Fred is correct in that an uncut floater casts, mends, and fishes in a more pleasing manner than one that has been cut and looped. Howeer, in my opinion, the tradeoff of a little less finess with the cut line and looped tips is worth it because you don't have to carry multiple rods to the river, nor do you need to unstring a rod and restring it with a different line to go from floating to sink tip.
Ah, me thinks we all need a personal ghillie to carry the extra rod.