Difficoult to judge a specific case but the basic idea is the following:
-stiff ski will be faster on flat, kick wax will keep longer, short traps will perform better on hard snow, but carving ability is bad.
-softer ski right the opposite, slower on flat, kick wax wears quicker, short traps perform worse on hard surface but carving ability will be better.
All the problem is the needed compromise.
Remember that narrow skis are tendentially stiff, and broader ski are usually also softer. Remember also that is you plan to ski in powder with traps, speed is reduced anyway, and traps performance will be similar between stiff and soft ski.
What is new on the market are the rocker skis. If you are interested in skinny ski, you should consider to go with at least some rocker, like on the new E99 or on the Asnes Gamme 54. This tecnology should improve the compromise; a relatively stiff ski with some additional control on carving in powder (no idea if the carving ability improves also on hard surfaces).
Regards