[Fate Stay/Night] Were Heroic Spirits always that powerful, or does the Grail enhance them?

Spoiler

Heroic spirits are manifestations of the people's imaginations, made solid by the energy of the Counter Force. This is why there can be heroes which have never existed in the real world, but still appear as heroic spirits, due to the people believing in them just that much. As such, a hero's power ebbs and flows with the amount of belief humans have in them, however heroes which were truly noteworthy in life will always have some amount of power, even if no one was to believe in them anymore.

What this seems to point to is that yes, heroes are boosted by the world spirit to be even more powerful than they were in real life, but not by the Grail, which is simply a vessel for mages to channel a great amount of energy. What the Grail summons are not Heroic Spirits themselves, but rather Servants, which are basically 7 empty "shells" (the classes) each with the soul of a Heroic Spirit inserted in them. This means that Grail servants are much weaker than their true Heroic Spirit counterpart, but it's unknown if this weakened shadow version is stronger than their base human form. Most likely yes.

Depends, there is a difference between heroic spirits and servants the servant class are container for the heroic so servants are weaker than heroic spirits. Now let's say shakespeare he was a normal humans so as a servant he is way stronger but takes someone like Hercules well he was probably stronger alive (though he did not have god hands but still nine lives because he invented it himself and all his other magic items) because Hercules as an archer would only be good in archery and as a Saber only good with a sword (when I say good it's as his best he could still use any weapons better than anyone but not as good as in a good class). Now differences with heroic spirits yeah they are more powerful than when they were alive because servants are only a portion of the Heroic spirits true power

Actually, the Heroic Spirits summon by the Grail are far more weaker than their real-life counterparts, since they rely on their Master's mana supply. Saber stated that with Rin has her master, she's almost at her previous strength.

Another example is that Counter Guardians have their mana supply provided by Gaia, witch is several magnitudes higher. A Counter Guardian EMIYA would be able to project several Excalibur class projects and nuke the crisis several times over as needed.