If Britney Spears can pull in $15 million to judge "X Factor" ... she sure as hell is competent enough to testify in court . . . this according to her self-proclaimed ex-manager Sam Lutfi.

If you recall ... Lutfi filed a massive lawsuit against the Spears family in 2009 -- suing for a whole mess of things, including defamation and unpaid management fees. For the record, Britney has denied Lutfi ever managed her.

Now, Lutfi wants Britney's testimony -- filing papers in L.A. County Superior Court yesterday, asking a judge to order Britney's conservators to make her available as a witness in the case.

Lutfi wants to grill Britney about alleged defamatory statements her parents made about Lutfi -- including that he drugged her and cut her phone wires to keep her away from her family. He also wants to ask Brit questions about an alleged management contract that guarantees him a cut of her fortune.

According to the filing, Britney's conservators have argued she is not mentally competent to testify as a witness -- so Lutfi included a DVD of Britney on "X Factor" in order to prove that's not the case.

Lutfi claims the "X Factor" footage shows Britney is lucid, articulate, and competent at the judges' table and therefore perfectly capable of being a witness.