Q1 = May I expect to begin planning at 10 knots with the new Code red 9.0 that I'm just ordering ? (assuming a good pumping ability)
A1 = Yes, the extra sail size (over) your existing 7.7m will force the most practical early planing out of the iS111.
Q2= Is there enough board overlapping In the middle of the range (18-25 knots ) ?
A2 = Just enough to be practical, but not too much extra ! You will find 6.7m is still good on iS111, but really the CR 7.7m has a bit too much power for general sailing on iS87.
Q3 = A 4.6 slalom sail would help to sail safely (I'm not talking about reaching ideal top speed, just surviving) on the 40 knots wind range?
A3 = Yes, for survival it will help for sure - and if the water is flat you will reach some crazy fast speeds in that combo, even though 4.6m is smaller than normal for that board/89kg combo. This one again depends on how "solid" the wind is at 40kts - if it's gusty offshore 40kts, (so 25-30 gusting 40) then likely the 5.6m is actually a more stable (if overpowered) power, if it's really solid 40 - then 4.6m+smaller fin (26-28cm) ..and hang on !!
Cheers ~ Ian