I think the best way to think of X (and Zero) power-wise is as a state of constant growth. It is observed from all ends of the X-series timeline, from MHX (Kuwanger) to Xtreme2 (Berkana) to XCM (Scarface), that X learns especially quickly from his experiences in combat. It is possible for an exceptionally powerful enemy to overtake him (Sigma, Vile, High Max, Great Redips), but he will prevail when given time.
X's "limitless evolutionary potential", however, does not refer simply to combat. His growth through combat, while exceptional, is well paralleled by Zero (noted by Berkana in Xtreme2). The potential which X possesses and Zero lacks, is his "evolutionary potential": his compassion, worrying, and general ability to empathize with people and forward the relation between humans and robots. Reploids, despite being free-willed, generally do not possess this capacity on nearly so deep a level as X, as noted by Zero in Z4 when speaking to Craft.
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