RE: new vs. old slalom shapes
Just can't resist jumping in.
I have been sailing slalom boards since 1990. Turned to short/wide just last season (Sonic 95).
I must note that rocker flat has evolved among narrow boards too. My '90 slalom board had a 50" flat; my '97 RRD 281 had a 30" one and '99 RRD 278 went "back" to 35", not that much for a 21 1/4" wide board with a 12 3/4" tail.
By sure new style boards with wide tails provide plenty of power and leverage over the fin, so that one can use a big fin for early planing and upwind and still use it at full speed; but efficiency is limited by tail and fin dimensions. Narrower boards just get into their own as the speed increases: the faster one goes, the more lift he gets from the narrow tail and the small fin. One must remember that lift is direct function of surface (so of tail width), but varies with speed square: at every small increase in speed, power is increased more than proportionally, so that one can use it to gain even more speed, and so on; and small fin and tail dimensions take quite a while before being felt as a limit to speed. The final effect is a feel of "unlimited" speed capability; a bit like that of a sporty engine as compared to a Diesel one. An Otto cycle motor increases efficiency as throttle is full open and revs go up, resulting in a feel of the engine wanting to run high; while a Diesel engine has similar efficiency at any gas position and speed. Just like narrow boards prefer high regimes, while wide ones are OK at any, but with a "dull" feel.