Hi twohand
The MidSpey will easily carry the Big Boy tips - especially the 10/11 (which is the equivalent of the 10/11/12 WindCutter). You will still need to remove tip 1 and tip 2. The 400 grain Big Boy will match best on that line, but the 300 won't be a bad selection either.
The only problem you may find is that with the longer head out, you will have more line out and the roll cast you use to get the sinking tip to the surface may not be so effective. If this is the case, just shorten the amount of line until you can roll the big boy to the surface in one go.
Hope this helps.
Tight lines
Simon