As I understand it, Crucio stimulates all the pain receptors in the body causing it to experience incredible pain. So theoretically, there should be nothing in the world more painful than the Cruciatus Curse. Is it correct to say that the curse is the most painful thing in the world provided it's cast with strong intent?

No, if it’s a correctly cast Cruciatus Curse.

It seems that Crucio can cause the maximum amount of pain that can be felt, if it’s cast correctly and the caster really wants to cause pain. When the Dark Lord casts Crucio on Harry, it causes Harry more pain than he’s ever experienced - and he’s been in several Quidditch accidents and had his bones regrown.

“Voldemort moved slowly forward, and turned to face Harry. He raised his wand. ‘Crucio!’

It was pain beyond anything Harry had ever experienced; his very bones were on fire; his head was surely splitting along his scar; his eyes were rolling madly in his head; he wanted it to end … to black out … to die …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

It’s also not possible to ‘get used to’ the effects of a properly cast Cruciatus Curse. The second time that the Dark Lord casts Crucio on Harry, Harry fares no better against it.

“Voldemort raised his wand, and before Harry could do anything to defend himself, before he could even move, he had been hit again by the Cruciatus Curse. The pain was so intense, so all-consuming, that he no longer knew where he was … white-hot knives were piercing every inch of his skin, his head was surelyl going to burst with pain; he was screaming more loudly than he’d ever screamed in his life –”
_{^{- Harry Potter and the Goblet of Fire, Chapter 34 (Priori Incantatem)}}

However, the Cruciatus Curse would only cause the maximum amount of pain that can be felt if it’s cast properly and the caster really enjoys causing pain. A weakly cast Cruciatus Curse would cause nowhere near that amount of pain. When Harry attempts to cast Crucio on Bellatrix, she doesn’t writhe or shriek in pain, and gets up and resumes dueling very soon after Harry’s attempted Crucio.

“Hatred rose in Harry such as he had never known before; he flung himself out from behind the fountain and bellowed, ‘Crucio!’

Bellatrix screamed: the spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had – she was already back on her feet, breathless, no longer laughing. Harry dodged behind the golden fountain again. Her counter-spell hit the head of the handsome wizard, which was blown off and landed twenty feet away, gouging long scratches into the wooden floor.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)}}

When Bellatrix cast Crucio on Neville, he writhes around screaming, and even after it’s over, he lies there crying at her feet.

“Bellatrix raised her wand. ‘Crucio!’

Neville screamed, his legs drawn up to his chest so that the Death Eater holding him was momentarily holding him off the ground. The Death Eater dropped him and he fell to the floor, twitching and screaming in agony.

‘That was just a taster!’ said Bellatrix, raising her wand so that Neville’s screams stopped and he lay sobbing at her feet.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 35 (Beyond the Veil)}}

The difference between a properly cast Cruciatus Curse and a weak one is in both the skill and intentions of the caster. As Bellatrix tells Harry, you need to want to cause pain and enjoy causing pain to cast Crucio effectively.

“Never used an Unforgivable Curse before, have you, boy?’ she yelled. She had abandoned her baby voice now. ‘You need to mean them, Potter! You need to really want to cause pain – to enjoy it – righteous anger won’t hurt me for long – I’ll show you how it is done, shall I? I’ll give you a lesson –”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)}}

There’s also a certain amount of magical skill needed to be able to cast Crucio, since Snape tells Harry he doesn’t have the nerve or the ability to cast Unforgivable Curses.

“Cruc—’ yelled Harry for the second time, aiming for the figure ahead illuminated in the dancing firelight, but Snape blocked the spell again; Harry could see him sneering.

‘No Unforgivable Curses from you, Potter!’ he shouted over the rushing of the flames, Hagrid’s yells and the wild yelping of the trapped Fang. ‘You haven’t got the nerve or the ability –”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 28 (Flight of the Prince)}}

Therefore, though it seems like the Cruciatus Curse can cause the maximum amount of pain that can be felt, it won’t always cause that level of pain - it only does if cast by a wizard who’s skilled enough to cast it and also enjoys causing pain.