You might be able to mesh two T300s, but 400m might be the edge of range, without adding additional external antennas.. Mesh would require a ZoneDirector. Consider also two P300, point to multi-point bridges, which can easily span your distance.
The exact distance is about 290 m according to " google map " .
My supplier confirmed me the feasibility without ZoneDirector with the new version of T300 ' s
Yes, Unleashed T300s do support Mesh now, and 300m is typically maximum distance for Mesh without external antennas. With good RF, you may be able to connect two Unleashed T300s at 300m by mesh.