I did a conservative back of the envelope calculation assuming a 50m x 50m x 0.5m flying wing covered in 25% efficient panels with a 0.5 coefficient of drag and a 0.3kg/m^3 pressure cruising altitude (a bit higher than commercial airliners) and an 80% efficient propeller. The speed came to 64m/s. The formula was v = (2*power / ((airdensity)(coefficientofdrag)(crosssectionalarea)))^(1/3)

The part inside the cube root might be fudged upward multiplicatively:
2x by reducing the coefficient of drag to 0.25
2x by increasing the length to 100m
2x by using multijunction solar cells
15x by tieing the 97000ft altitude record of Helios where the density is ~0.02kg/m^3
The combination of all those fudge factors is 120x, and the cube root of that is 4.93, so it seems an upper bound on solar airplane speed would be 64m/s*4.93 = ~320m/s, which coincidentally is almost the speed of sound.