WAAS improves accuracy by compensating for large scale errors in the GPS system. For example, if a satellite is off course, or if there is a big storm in the ionosphere, these affect the accuracy of a position lock in a predictable way. The WAAS satellite provides information that the GPS uses to correct and negate these errors.
But WAAS does not help errors that can come from other sources. For example, if you only have a small quantity of satellites visible, or if there is local or cockpit interference, or if you have a week signal because you are not using an external antenna, all these things affect accuracy.
So it's very hard to assign specific number to a question like this. Bottom line, WAAS makes it better but it's not a guarantee of accuracy. You need to factor the signal strength. And even with full bars I wouldn't fly a precision approach with a hand held unit - you really need an externally mounted and professionally calibrated GPS antenna to ensure accuracy.
-Walter