GCD questions

[Mhaeric]

So, let say I stand still and cast 3 times the same spell in a row, is there any difference between these 3 spells?
instant, recast 2.5, potency 300
2sec, recast 2.5, potency 300
2.5sec, recast 2.5, potency 300

The first spell you can weave 2 instant ogcd abilities before the next gcd without clipping it, the second you can only weave one without clipping, and the third you can't weave any without a slight bit of clipping. That's in addition to the additional mobility the extra time provides if you do decide you want/need to move.

Same goes for the healers, does this mean that if I spam CureI, I effectively can't do nothing between everycast for 0.5sec?

With healers there's also the extra benefit of getting the heal out faster. With dps it doesn't matter quite as much when the damage actually lands, but when it comes to healing a 2s cast time can result in less stress over a 2.5s cast time and an instant heal is a godsend. Also the 2s cast time is helpful with ogcd instant heals or other abilities like AST cards or mitigation tools like Virus. E.g. Benefic --> Esssential Dignity --> Benefic has zero clipping because the essential dignity fits neatly within the 0.5s window between the cast time and the gcd.