I think getting a iS90 to go with your iS101 is not good advice, not a real gap between the two boards. Would not make sense to go iS101 & iS90. Please, please do not waste your money on bad combo
iS90: Wind & sea gets big, you will be last in race because no control = no speed
If you had a iS111/107, then yes the iS90 would good. If you really worry about wind going, then the iS87 will be safe bet than iS80 (but I think Remi is right, iS80 best for you). For a 87 litre board, it is pretty big board. It has a powerfull tail and it will take up to a 38cm fin if the winds are marginal to keep you powered up so do not worry that this board is too small, but if its marginal, you will be on your iS101.
iS87: Wind & sea picks up, you at front of the pack because good control = fast speed
The lighter iS90 carbon will still much feel bigger than the heavier iS87 wood for sure.
Were do you race?
Hope that makes sense