Not the SB team here, but like you I have wondered what the "speed" difference will be between old and new.
Reading between the lines, I think that some people struggled to get to best out of the iS because it is very short. Normal slalom riders with an active style did not find the iS hard to sail fast in light winds.
On paper, the US147 extra length will do the following:
More passive planing.
Better planing through lulls.
I also "think" that from a top speed point of view, the sharper rails will counter the extra length = slightly faster top-end (just a guess).
I tried the old iS150 (2008) for a few weeks and I liked it a lot with a 9m and 10m in super light winds. I dont think that you will be disappointed with either boards, but I think the US147 must have the edge on the iS150. It seems that SB always moves forward not backwards