Silence might ne more to do with "how do we answer that one..."
rather than the iS155 being "wrong".
Basically the iS155 (iS145) will be an easier board to sail fast off the wind in powered and choppy conditions, which gives potentially a quicker solution HOWEVER you do also need to consider a bunch of other variable like rider skill/weight (ability to EFFECTIVELY make use of bigger fins or wider tails - or not) and also the critical variable of "how powered up" the comparison is - because in marginal conditions (say light to mid 11m) then (esp a more FW experienced and/or heavier rider) MAY be more efficent (read : quicker) on the wider tail (and/or greater fin) of a Formula. In powered 9.5m (or windier) conditions, most riders would probably prefer the iS155 for comfort/control/safety, even if the ride speed advantage was not quantum over FW.
If, at the end of the "quickest off the wind" comp, you also had to nail a fast jibe (or series of them, like maybe a slalom race), well then the added on water handling advantage of the iS155 would be another plus.
In general, the majority of riders see (perceive) slalom to be a more hi wind / hi speed discipline than "fast FW conditions" and simply don't consider a 150Lt board to be a "slalom" solution c/w 100Lt rockets.
Realistically, the wind range that a 150Lt slalom is going to be competitive over a well tuned/well ridden 111/122/133 is also pretty narrow. Having said that, there are many locations that don't enjoy full on slalom conditions and many riders who may be heavier - but still want to go "fast" , or even prefer a more passive technique - for these guys the iS155 is a valid option.
That may or not have answered the question,
but I hope it explains why
Cheers ~ Ian