red-haired crew, no devil fruit, still really strong? I've heard on many online forums that the red haired crew is the only crew with all Human Pirates. What exactly is that makes the crew so exceptionally strong and the captain -Shanks- being one of the four emperors.
They've been portrayed really strong, in the Marine ford arc, several admirals also Blackbeard and his crew were afraid of fighting them, Admiral Kizaru -who also seems really strong- was shut down just by a look from Ben Beckman.
What exactly is their extent of powers being normal human being and have they shown it in any episodes?
Right now we know very little about their abilities.  We do not know whether they have any Devil Fruits. Even Shanks, who we know did not have a devil fruit when he lost his arm, could have eaten one more recently.

That being said many strong characters don't have devil fruits.  Silvers stopped Kizaru and was shown swimming later. The majority of the Strawhats still do not have Devil Fruits including 2 of the 3 strongest fighters.

Haki (and possibly Sea Stone) seems to allow non-Devil Fruit users to fight evenly with Devil Fruit users. Shanks has been shown to have incredibly powerful Conqueror's Haki to the extent that he damaged Whitebeard's ship with no visible effort or action.  It is likely several others (especially those who halted admirals) have Haki too.

We know that Ben Beckman is the smartest person to come from the East Blue which puts him above both Nami and Captain Kuro (man of 1000 plans). Yassop is a powerful sniper like Ussop (who is more skilled that a platoon of WG soldiers despite appearances).  Rockstar had a bounty of 94,000,000 Beri.

We literally only know the names of 5 of Shank's crew (the above and Lucky Roo) though we have seen several more.  There is no telling what insane abilities each member of the crew might have when we know so little.  The few we do know were able to at least intimidate marine admirals.  There is more out of universe (fan theory really) evidence that one day Sanji will confront Kizaru and Zoro will confront Fujitora. I don't think anyone would doubt that they are powerful crew mates and will likely never rely on devil fruit powers.