The wing behind the fuselage in a slip will get less air moving over it, therefor generating less lift. I suspect (gut feeling) that the possible lift generated by the fuselage does not compensate that, so I suspect you end up with less lift in a slip.
Anyway, the stall speed is dependent on the angle of attack, not on the amount of lift produced. If you keep your wings level, and perhaps compensate the extra drag and loss of lift with extra engine power, then you should have about the same stall speeds. But to keep the wings level, you have to compensate for the roll effect caused by the yaw, increasing the angle of attack of the wing behind the fuselage, causing that wing to stall sooner.
You have to take into account the shape of the wing: swept or not, dihedral or not, that all have an influence because of how the air will flow over the wing in a slip.
My best guess is that your stall speed goes up in a slip.