Replacing VOCA with a Laptop?

tomschuring asks: "A friend of mine has Motor Neurone Disease (MND) and because of it, he is using a Voice Output Communication Aid (VOCA). When I heard the quality speech of the portable device I was less than impressed. My Mac is much better at it and at least has a few different voices (like one with an English accent) to choose from. Has anybody used a laptop for this purpose? What text to speech engine are you using and what are our impressions? Is there predictive text software available for this purpose? Is the startup time and battery time acceptable for this sort of application?"