I'm reviving this thread with an important addition.
Something I've been suspecting for a while has been confirmed recently. Guardian power levels canonically vary even for individual Guardians. This has been implied before and would also explain away some situations that seemed like inconsistencies before. However, as shown by the feats that give them their current ratings, Guardians do seem capable of performing on specific levels pretty consistently. The second image also makes it clear that general gaps in power between Guardians very much do exist.
This is a straightforward example of that. For most Guardians there's not a clear starting tier, so we'd only list their higher ends.
The relevant quotes:
"Guardians insist on treating ‘power' as a monotonically increasing value with a single dimension. Guardians will casually create ordinal ranks of the ‘power’ of their comrades when, in truth (but not in fact—Light is not constrained by facts) capability with the Light is not only a multifactor landscape but strongly and stochastically influenced by circumstantial, personal, and esoteric variables. The Light is, by definition, paracausal and obeys a logic different from physical concepts like charge, heat, or angular momentum. Any analogy to these physical measurements of power' will fail to capture the true efficacy of a Guardian."
"IKO-006 ranks in the upper fraction of the 99th percentile of assessed Warlocks on most available metrics of precision, restraint, and raw power. She is, in simplistic terms, a fifth sigma Guardian: 1 in 3.5 million. Given that millions of Guardians have been activated over the centuries since the Collapse, and assuming that performance of Guardians on these metrics is normally distributed, we would expect about ten Guardians of similar power to have existed."
So my proposal is that all Guardians get a Varies tier while listing their current ratings as their peak. E.g. "Varies, up to at least 5-C, likely 4-B" or "Varies, up to Massively Hypersonic+" for their speed.