I feel like this might be really pushing the T300s to their limit depending on the bandwidth requirement for the link... I only cover around a 200m area and even with clear line of sight around the edges you can start seeing throughput drop due to lower RSSI noticeably even with 80MHz channels and good RF.
Perhaps a T301n or T301s might make this more realistic, but the P300 pair is really designed for this job and appears to be priced attractively relative to buying two T301n's.
Rostom, out of curiosity, why the insistence on using the T300 specifically for this job?