Trading Energy for Speed and Skill
One way to introduce more flexibility into standard spellcasting is to allow casters to exchange extra energy for special benefits. Below are two examples; see Adjustable Spells (this page) for an additional option. All such rules give magic-workers who have lots of spare energy (high personal FP, piles of Powerstones, etc.) a considerable advantage. [...]
Faster Casting: Wizards can reduce casting time by one second per 4 extra energy points spent. This option can’t reduce casting time below a second, and isn’t available for ceremonial magic. Increased Effective Skill: The energy-for-skill tradeoff of Ceremonial Magic (p. B238) applies to all castings: +1 to skill for 20% extra energy, +2 for 40%, +3 for 60%, +4 for 100%, and another +1 per additional 100% of the required energy. Such skill increases affect the roll to cast the spell and the Power of enchanted items, but they don’t reduce casting time, energy cost, or ritual requirements. A further option for Missile spells is to give +1 to the attack roll per extra energy point spent.
[Source GURPS 4th Edition Thaumatology p.39]
Does this mean that if a Spell costs 1 to cast and I have skill 20 which reduces casting cost by 2 that I can "virtually" spend +100% on the spell (i.e. 2FP - 2FP = 0FP) to get the +4 bonus to the effective roll?
The question could also be formulated as: Is this extra energy part of the casting cost?