No he shouldn't, if you are of the opinion that admirals/yonkos are relative in power (even if any yonko can beat any admiral or vice versa) and that they are close to PK level (high diff).
Luffy could always stomp or at worst low diff Sanji, that's more evident ever since the post skip. He's not gonna fight someone relative to Akainu (assuming he's the final opponent for PK level Luffy) or someone that can give a good fight to a PK level Imu. Akainu or Imu would need to be way stronger than the admirals for this to work.
His best bet vs an admiral would be in a team battle. Kinda like how the SN fought the Emperors, but with fewer ppl. Sanji + Yamato vs Greenbull? Or maybe with Jinbei instead, goes well with their fire-water theme + they teamed up in FI.