Q:

Was Incursio really weaker than Grand Chariot, or was Wave just delusional?

Wave states with great confidence that Incursio is weaker than Grand Chariot, because Incursio was a prototype for Grand Chariot, etc.
The early clashes between Wave and Tatsumi can lead the viewer to such conclusion, but Incursio's has the ability to evolve (from the beast it was made of), while Grand Chariot doesn't.
If you watched to the finale, we can see that

 Tatsumi causes Incursio to evolve so much during the fight against Shikoutazer that it even gains the ability to fly. While it is enough to defeat the emperor, unfortunately not enough to save Tatsumi's life.

Considering this, I don't think that Incursio was the weaker one. Is it so?

A:

Currently there is no actual way of knowing this, especially not in the anime variety as this bends off from the manga's story quite a bit.

Wave states that Incursio is the prototype of Grand Chariot, meaning it was created after Incursio. (ch 18, p 36) (ep 10)
Its full capabilities, however, have yet to be revealed and it is therefore unknown if it is more powerful than its predecessor, Incursio.

But assuming that Incursio's Adaptation & Evolution are limitless, we can most likely state Incursio to be the stronger armor. But as we get to know in the manga (do not believe this is shown in the anime) it also has huge side-effects
spoiler from the manga

 However because Tatsumi forced Incursio to evolve rapidly to increase his strength, Incursio started to manifest in one of Tatsumi's eyes. After being examined by a doctor, he reveals that the armor has started to bond with him. He also states that after a few years Tatsumi could use it without overdoing it like Bulat. Now at the risk of being devoured by Incursio, Tatsumi can only transform 3-4 more times before Incursio consumes Tatsumi.

A:

Rather than evolving, Incursio adapts to its user. Tatsumi just happened to have huge potential, which lead to Incursio evolving along with him.
If we think back to the days where Incursio belonged to Bulat, its only ability apart from the obvious strength enhancing one which it shares with Grand Chariot, is its invisibility. Bulat had owned Incursio for many years longer than Tatsumi ever had.
Also, we know next to nothing about Grand Chariot's own abilities. For all we know, it might be the same as Incursio, only much younger and thus less "experienced".

