I think Mozart with a computer would have been farther away from the pop-click-whirr-beep repetition-based electronica and closer to groups like Tortoise or STS9 that use electronic music to complement instrumental music, rather than relying solely on the computers. Like others have said, he would use all the tools available - which includes computers and synthesizers but which also includes the same orchestral instruments we’ve always had.
He would have spent months creating long Flash animations of lolcats. No, really…
The answer is “We Built This City”.