Hi Ola, thanks for the reply. That certainly is an interesting post which has both explained why I was wrong and confirmed why I was right
Ian Fox comments that the narrower board is more suitable in GPSSS conditions of very flat water which I think confirms the traditional view of tail width being instrumental, along with rocker line, in the overall top speed.
However, as we all know, these boards will generally be sailed in less than perfect conditions, in reality the majority of people sailing them (me included) will achieve higher speeds through the ease of use of the board and I think it is generally accepted that the iSonic range has been so successful due to this user friendliness.
Whatever the details, if the board is as good as my iS111 I will be more than happy!