He was huffing and wheezing in a panel it clearly did take stamina out of him. I don't think he got that much stronger unless he has a power up all he learned was if he used intense enough Haki he could break through Devil fruit powers. Point is Big Mom took on Awakened Kidd+Law and had them beat and turned around instead of finishing them off compared to BB who's struggling with Law alone. Right now to me it's obvious that Big Mom is stronger than the current Blackbeard.