According to the Merriam-Webster's 11th Collegiate Dictionary (online version), that spelling and pronunciation, while they are common and go back to the 16th century, are non-standard.
It's mischievous, not mischievious.
This is the place where Richard Mateosian comments, sometimes nitpickingly, about questions of writing style, grammar, and vocabulary that occur to him as the words of others impinge upon his consciousness.
Then there's jumbo, which we don't know how much of JPM's business is composed of that.
You can convert that into a grammatically correct sentence as follows:
Then there's jumbo; we don't know how much of JPM's business that comprises.