Yes and no... Caster can see or feel the weave or the forming of spells as their being casted, this is like a tingling or extra sensory caster have, even more so IF they have a detect magic spell active... Removing voice, gestures and components is to avoid non-casters ability to pick up on them, handy when you use magic in crowded settings, or to use spells when for example a silence spells, or tied...
i think the difference between a sorcerer and a wizard is that a sorcerer uses his/her inner power whereas a wizard uses the weave.
i think a sorcerer still can use his/her magic if there is an area without the weave, unless the area is an anti-magic area or something that can absort magic.
this means if a wizard casts a spell with time "immediate", but the wizard still need seconds for voice and gestures to touch the weave. other casters can sense it and "react" a "counter spell".
but, a sorcerer cast a spell with time "immediate" with "subtle spell", this is the true "immediate", because a sorcerer needn't spend time for voice and gestures, in addition, a sorcerer needn't touch the weave, since a sorcerer is a "magic fragment" form some god/goddess. this means other casters don't have enough time to "sense and react", they only can "sense" a completed spell suddenly appears and too late to react stop the spell.
