Courtesy of a link from the McLaren website here's a short video of Ayrton driving a Williams, McLaren and Toleman in closed season testing at the end of '83. I'll let you watch it before making any comments.
I do like Franks comments to the interviewer about it not being Williams policy to sign young drivers. That seems very odd in this day and age. I suppose that as Frank already had Rosberg and Laffite under contract for 1984 there wouldn't have been a place for him in the team unless he was signed as a tester.
The thought of Senna ending up in a Mclaren for 1984 is also another interesting one.