Tbh Luffy spent the last 4 chapters doing nothing but eat food for giants, and he was already in his obese mode 2 chapters ago with him saying they can't win if he doesn't eat more (implying more food = more power)
Maybe Oda will make G5's strenght linked to how much Luffy ate from now because it is tied to the power of his imagination and Luffy thinks food is what gives him strenght lol.
Also Luffy only needs to survive against Imu, Imu's time implied to be limited and Loki alraedy been taking on his time