A few things:
* you will only get a max of 300Mbps since you can only use one radio for the mesh link. Tha radios are 2x2:2 so you can do two spatial streams at a max of 150Mbps for each chain.
* while the 'show ap mesh topology long' will give you a tx/rx rate, it only reflects the last exchange, and if your link isn't highly utilized, it may reflect a slower than accurate link speed.
Outside of reading the guide to identify best practices, priper antenna height and LoS requirements, etc, if you can post the distance and the types of antennas you plan on using, I can give you an approximate expected throughput.