I was flipping through my pdfs, and I noticed that instant spells are handeled much differently than I remember, in particular Stun Relief.
The actual rules are often very different from what folks remember. For example, I would feel safe in guessing that you didn't use the core RM2 tactical system (i.e the Phases) - http://www.ironcrown.com/ICEforums/index.php?action=tpmod;dl=item578 - which is actually where the 75% activity for an "Instantaneous" spell originally came from.
In fact, many people didn't realize that when the original authors used the word "Instantaneous" in regards to spells, they weren't actually talking about how long it took to cast them, but referring to the fact that none of those spells EVER required preparation rounds, and thus could ALWAYS be cast instantaneously (i.e. right away)
As I recall back in the day when someone got a "stunned" result on the crit tables, Stun Relief would automatically fire off. The character could then act as normal, that is to say he could attack. He just couldn't cast a spell since he had already cast one that turn.
More than likely, your group used house rules, and quite possibly used its own tactical system rather than the one from the core books (many people, myself included, did this).
As others have suggested, there is always Option 5.2 on page 30 which basically reduces the total activity of an Instantaneous spell to 10%...
Now it seems that RMC uses a percent action system, and casting an "instant" spell takes 75% of your action. That doesn't seem very instant. In fact, according to the Actions table in Arms Law about the only thing you can do with only 25% of your action remaining is stand up from a chair.
Reread page 30 of the RMC Spell Law. Casting the Instantaneous spells only takes 10% activity, the other 65% activity being a recovery period from the instantaneous casting. So yes, while it takes 75% in total, the Instantaneous spell goes off first (since it is considered a Short Action).
So the question becomes "What good is Stun Relief?"
Stun Relief isn't in there so that you can get rid of the stun and attack in the same round. That was never its purpose. Its purpose was to get rid of the stun, period. Being stunned makes you easier to hit and hurt. Having the stun go away, even if you cannot attack in the same round, is VERY beneficial.
Additionally, Stun Relief is (in almost, if not all cases) a subconscious spell. Which means that so long as you are the same level or higher, and have the PP available, the spell will go off automatically, without you having to worry about too much (i.e. it only fails on an 01-02).