Actually, we have information on Mikaela’s power level as of before he drunk Yuu’s blood and became a complete vampire. The City Guard he was a member of is considered to be about equal in power to lower ranking progenitors, and he is said to be one of their most powerful fighters. So, imo, Mika could beat Mel Stephano, for example, and maybe stand a good chance against Horn or Chess at that point. After he drunk human blood, it is likely that he became a rank or two stronger.
What we know about how vampire strength works is a. it is basically predetermined by the quality of the blood that turned them, coupled with a person’s own potential and talents (from Kagami’s Q&A), and b. it grows stronger the more they live (Fanbook 8.5, page 122).
It is said nowhere that a progeny would become as strong as their sire, but considering that quality of blood is so crucial, it certainly is a natural thing to assume. So yes, I would say that the upper limit a vampire’s power can reach is about the same as their sire’s power. We even have some indirect proof of that (like Mahiru stating that she obtained power equal to that of a 3rd progenitor (that is, Krul’s) when she attacked Saitou). Which would mean that Ferid and Crowley’s upper power limit is the 2nd progenitor power level, and Mika’s the 3rd progenitor power level. Edit: In the light of volume 2 of the Michaela novel, the blood weakens with every next generation, first by 5 ranks, then by 6 ranks, and then even more, it seems: a 2nd progenitor’s progeny can only be a 7th rank at best, a 7th progenitor’s progeny becomes a 13th rank at best.
Somewhat unrelated but, my copy of the novel is still a week away (it’s gonna be a long and painful week for sure), but I read the spoilers, yes. I’m kinda happy that Ferid is not Crowley’s sire, tbh. For one, 2nd is certainly better than 7th, gives Crowley more potential to work with, and for two, less connection to Ferid. If they’re “brothers”, so to speak, it is likely that point b applies in full, and Ferid is stronger precisely bacause he’s like a millenium older. It’s intriguing how Ferid apparently turned neither Crowley nor Mika, arranging for mightier sires for both of them, likely precisely because they’re Michaelas, but with him reaping the benefits and using them for his purposes. Now I’m dying to know which 2nd progenitor turned Crowley: was it Saitou? Gilles? maybe that Raux guy turned out to be a 2nd progenitor? or someone else we don’t know? Can’t wait to find out!