So an injured old man performs better than Gear 5 Goofy ?
I really hate Gear 5 more than anything else now
There are instances where you can't just take 1 scene as a complete appraisal.
We know Imu was surprised to see Gaban, so it could have been a surprise that he was taken off-guard.
Huge difference between taking someone off-guard and landing a decent blow... and then fighting someone who has their full attention on you - we need to actually see what happened.
Power-houses in One Piece can recover quickly too, this is not a new thing in One Piece, we see it all the time. Garp almost took out the Blackbeard Pirates with a fatal wound.
Luffy also has not been outperformed yet as far as we know - you can't really compare what someone does with a Sword to a fist.
Swords WILL ALWAYS cut, a Fist will only do blunt damage, a Sword is a far more fatal weapon - fatality of an attack does not define outperformance especially when it's 2 completely different modes of attacking.
There's literally not enough information to even decide that Luffy has been outperformed, how can you judge that from a statement or 2... it's one thing if he cut Imu as he uses blades... it's a different thing if he straight up killed Imu already.
Imu is also able to regenerate, so this attack while it would hurt, it is unlikely to be the conclusion of the fight or permanent damage.
Sounds like you're overreacting, we don't have the full context to see what exactly has happened - even spoilers are usually missing a lot of important details. I'd personally reserve angst and much judgement until the actual chapter is released, at least wait for a summary.