I can assure you that from my singular anecdotal experience that a diagnosis does not imbue economic and financial benefit.
You don't even get social benefits, no one excuses your behavior just because it has a label. You get told it's your fault for not managing your disorder properly. Have you seen how we treat people with visible, obvious, undeniable disabilities? Like shit.
If they're faking it for benefits they are REALLY committed to the bit.