Currently, MechJeb doesn't take the stock thrust curve into account when calculating SRB burn time, resulting in a burn time value that's off by a large margin. This causes issues with PVG, particularly all-solid stages, which show a much shorter burn than they actually have. Could the burn time be calculated based on the curve instead?

No, I'm not going to put an integrator into the fuel simulation.

The dev version uses the 50% point of the SRB curve which is much closer, that's as good as it is ever likely to get.