I usually assume that all magic, whether Earth Pony, Pegasus, or Unicorn; presumably if they reached some extreme level of peak control over their magic, and it was exercised well enough to give the magic enough kick…
…Then the difference between the races would sort of petter out at the high end. Somewhere just before Alicorn.
Think of it like a bell curve. An Earth Pony that is horrible at using Earth Pony magic is going to be roughly equal in skills and abilities as a Unicorn who is equally bad at Unicorn magic. (See Sweetie Belle VS Applebloom.)
Meanwhile, an Earth Pony that has mastered Magic, and even has it as a special talent, could theoretically perform feats of magic similar to a master Unicorn. With the last difference between the two being merely a matter of fluidity in use of those skills.
In the mid range between those two extremes, the differences tend to maximize at a point on the graph corresponding to the average pony’s abilities.
Looking at our best examples of magic in each race: Pinkie,Rainbow Dash and (Pre alicorn) Twilight, we get a good idea of mid range differentiation, as well as some convergence.
Pinkie can perform great feats of speed, rip through space-time, and has mild physical premonitions. The ripping through space-time has been done by twilight Sparkle as Well (Magic Duel.
Rainbow Dash can bust through to sonic Speeds, letting off a decent explosion. Which at one point made her mane incorporeal like Celestia’s. So far, Twilight has not been able to replicate a similar feat, but Pinkie has come relatively close.
Twilight is your Typical “Red Mage”, Jack of all trades, master of only the one that governs all of them. But this is mainly done through manipulation of the structure of the world around her, something that has only been alluded to being done by Zecora, albeit mostly through conduit alchemical substances, save for her glass trick…so … Hmm?
We can guess that this point, for at least individual ponies, is the point of greatest Differentiation. So for the next half, lets theorize on what their potential for magic at different levels are.
Unicorns: As we can all assume Unicorns have the most stable growth curve of the three pony kinds for use for magic, or at least it’s practical applications. At the start, it gives some of the most useful abilities, such as Levitation, and other fine manipulation abilities. Plus, once they have discovered their talent, they quickly gain a good tool-set of abilities heavily specialized for what their talent is.
The only real reason that their magic skills peter off in ability after that, is more because they rarely have the reason, or drive, to push the boundaries of those abilities.
Earth Ponies: Unlike Unicorns, Earth Ponies have to wait until later in their curve for more practical abilities. Their First practical abilities are usually general but flexible tough, being some large boost to some physical ability, such as speed or strength, generally suited to what tasks they need to do. Which can carry them through most day to day challenges as well as a Unicorn of the same level.
Some of their more earth shaking abilities though, they don’t show up until much later. Such as any theoretical “earth bending” that may go on. However, theoretically if they manage to reach that far in the curve, their magic would be strong enough for their abilities to be devistatingly powerful. Albeit clunky and uncontrolled.
Pegasi: The pegasi growth curve seems to be erratic at best. As most low end Pegasus already have the gift of flight granted to them fairly early on, as well as weather control, which is undeniably useful, despite being something that requires preparation.
Higher end Pegasi? They practically get freaking energy fields around them, and some get steel sharp wings.
Our Highest known tier of Pegasus (Rainbow Dash) has finely tuned all these aspects into a weapon, the Sonic Rainboom! And a side effect was her mane becoming incorporeal.
That last bit, is our only clue as to what a Pegasi’s progression beyond Rainbow Dash would be. Becoming one with their associated energy, possibly to the point it bends to her will or emotional state. Similar to how dark clouds swirl around Luna while she’s angry, yet she doesn’t seem to be using Unicorn magic.
Side Note: What can we say about dark magic? Well, from what we’ve seen of Twilight, it’s something that doesn’t only work through the horn, but also flows through the eyes at the very least.
Presumably any pony, regardless of kind, who’s able to tame their anger and sadness, could use a large range of magic through that power.
However, it would take a small subset of pony to do so without winding up being the kind that becomes a tyrant.
Such a pony would have to have anger against injustice, sadness from loneliness or fear at the prospect of losing their friends to something violent, and a strong will to continue living; despite the emotional damage such a state of mind causes.
It’s possible to find, yes. But so far, this small subset is rare in any reality. And… I’m actually surprised I’m able to both empathize and sympathize with that state of mind as well as I can.
…But then again, Luna’s practically the poster-child for dark magic in some of its forms, both before, during, and after Nightmare Moon; and a lot of people can relate to her. So maybe I’m underestimating here.