As a recent owner of an Isonic 111 (sailed 6 sessions) I can say that the 42 cm fin is not suitable for speed while powered with a 7.0 sail . I use the 111 with a 7.7 Severne code red sail in the 12-18 knots range with the stock drake 360 fin (good upwind performance, no spinouts, high downwind speed).
The max gps speed I measured is 31 knots in far from ideal speed conditions (gusty-medium chopp).
I guess that for a 6.7 or 7.0 sail, when powered at the 15/20 knots range the ideal fin size is between 34/36 cm, and a skilled sailor would reach 34/35 + knots in flat water.
As your IS111 comes equiped with both fins (42 and 36) you can try this without buy a new one for the first trials, then you can try
more speed dedicated fins in order to improve your results.
Please tell me your experience later