I'm not sure if this is a rules question or an advice question, but here it goes. Let's say you are a spontaneous caster and you have taken your turn. You have an ability that lets you sometimes cast a spell that normally uses a standard action as an immediate action, that is to say, you are not casting it on your turn, but on someone else's turn. You cast your spell as an immediate action and you chose to add a metamagic feat to the spell, which makes the casting time a full round action. When does the spell go off? What happens when it comes back around to your turn again? Do you get a turn? Do you get a partial turn? Are you still casting the spell?
It depends on the rules text of the ability in question. There's no general rule covering all cases of "immediatifying" a spell.
Counterspell Mastery (Su): At 6th level, you gain Improved Counterspell as a bonus feat. You may attempt to counterspell an opponent’s spell once per day as an immediate action (instead of a readied action). You must use a spell at least one level higher than the spell being countered to use this ability. You can use this ability once per day at 6th level, plus one additional time per day for every 4 levels beyond 6th.
That helps, thanks!
Counterspelling is a pretty special case, as it involves readying an action. Metamagic feats are irrelevant for the purposes of counterspelling, so I'm struggling to think of reasons to apply metamagic to dispel magic when it is being used to counterspell. Maybe Stylized Spell if for some reason you don't want people to realize you're the one countering it...?
As best as I can read it, a spell's casting time has no bearing on whether it can be used as a counterspell. You use a standard action to "ready to counterspell", and if you identify the spell and are able to cast it, you simply do, countering the spell. The casting time of the spell being expended as a counterspell is immaterial--your readied action goes off, you cast the spell as a counterspell, the spell is countered (or not, if this is dispel magic and you fail the dispel check), your initiative changes, and that's it.
In such a case, my answer to "does it still change the casting time if I spontaneously add a metamagic feat to my counterspell" is "doesn't matter".
Using heighten spell with improved counterspell sounds handy for a spontaneous caster who needs to rummage through their spell list to find the correct school to counter, but then needs it to be high enough level.
The real rule question is if a heightened spell can be used to counterspell normally, if it can then it should work with this special case of countering as an immediate.
That makes sense, and it should work.
To be honest I was more interested in how it would affect the characters next turn. For example, if you take your turn and you use an immediate action when it is not your turn, when it comes around to your turn again, you don't get a swift action on your turn because you already used it, so to speak. I was wondering if there would be a similar case here. Presumably you wouldn't get your swift action for the same reason, but I was wondering if you would lose, say, a standard action, for example, because you are doing a full round action outside of your turn.