I've fitted a 26.0 mm bar into a 25.4 mm open face stem with CARE so as long as your not ham fisted its OK. Make sure there are no sharp edges where the half circle of the clamp meets the open clamp face. A sharp edge here may dig into the handlebar when tightened so remove it with a file or find another stem. Loosen the clamp right off when you wan't to adjust the handlebars so as to avoid deep scratches in the handlebar which can act as a stress riser and carefuly and evenly torque down the handlebars.
OK, one more thing. I measured my 26.0 mm bars as an actual 25.9 mm. If your bars are actualy 25.8 mm then they will fit without any issues anyway. If they are actualy a tiny bit bigger than 26 mm then things may be different.
Regards, Anthony