If we disregard aero arguments (applicable only really at very high speeds) then the only use of the extra volume in the front of the 100 would be in extreme sub planing and uphauling.. ( not really "real" slalom race issues I think you will agree
At any sort of normal stance and minimal fwd speed, the upper side of the S100 nose area is well clear of the water and not acting in any functional way (OK, generalisation, but you get the idea). Simple test for that is to thin that section out (thickness) wise and go test.
With iS101, there is a much lower nose scoop/rocker, and combined with the extra volume (and width) distribution at the back of the board (driving/keeping the iS101 tail more "up" at any sort of fwd speed c/w narrower , thinner S100 tail), this all gives the iS a flatter (less nose raised) angle on the water( even in sub planing) and can appear as you described. Taking a slightly more aggressive, rearward stance on the iS101 can compensate for some of that.
More subtly, but importantly the real difference is more likely in the rails (as mentioned in my above post), where sharp(er), boxy rails on the S100 have given way on iS to a (slightly) softer, more tucked profile (which improves top end control/rideability/confidence etc). In isolation, that could be considered to be a potential for reduced "marginal" performance and/or early planing ability.... Especially by riders who had evolved good rail "edgeing" technique on older, narrow, sharp railed slaloms to get going (earlier) in marginal/sub planing conditions (might sound odd to those who don't, but it's real, they exist and yes it's an art). However (the big BUT...) when considered as part of an overall package, adding (back) in a wider tail (increased planing surface area) and increased ability to handle bigger fin area (in marginal conditions) , any "loss" due to softer rails can pretty much be negated - and an overall better package achieved (by most measures !!?? - especially overall range, not only of the board itself, but a board that is more tolerant of sail size range, that's to say can more adequately handle overpower situations for example).
Again, to get the most from this new style really does require a few minor style/technique (and even tuning) adjustments - and I agree the initial "feel" might leave hard rail addicts a bit underwhelmed in marginal conditions. Pretty quickly offset when viewed in overall context !
Does the iS96 plane earlier than S100 ??? Marginal and decided on technique. A more powerful, rear (and fin) focussed technique will show less disadvantage to the iS96, whilst a smoother slide forward, rail edgeing plane up technique may favor S100.
We're pretty strong advocates of providing as much information as possible, and YES, Steve, I agree in general more images 3D animations etc all look good and can help, but for sure the subtle detail variances that really relate to this issue (and similar) would not be so easily portrayed or observed in even the best 3D animation. And even if fully visible, "interpreting" these into comparative performance/behaviour is an even further descriptive challenge (yes, happy to stop writing anytime
Cheers ~ Ian