I saw that SI couldnt increase the speed of magican?But how about weapon speed?Will it affect? I remember F/P uses Evil Wings in 3rd job because it will let Explosion faster.
SI is specifically for weapon attacks, not spells. All SI would do is increase your melee swing speed with your wand/staff
Short answer yes. (Qn in title) Note: Spell booster increases your casting speed to the max regardless of weapon speed.
SI can affect the FP mage explosion skill since it's based on weapon speed. It will also make your melee swings faster. It doesn't affect other spells though