You could get 300Mbps theoretically.
Take this topology for example: PC1--CPE510_AP)))))(((((CPE510_Client--PC2
Packet send from PC1 to PC2 is encapsulated by CPE510 and when it comes into a wireless frame the size is nearly double originally one(According to experience the wireless frame expense is approximately half of the whole frame). That means, if wired data rate is 100Mbps, the wireless speed should be at least 200Mbps.
Besides, data loss as a result of other factor such as obstacle, interference occupies part of speed. So the real wireless throughput is over 200Mbps. In this light, the advertising 300Mbps is significant.
Of course, 10/100Mbps port could be bottle neck of this device.

300Mbps is Physical layer maximum wireless speed.
There are many overhead and data loss in the open air. The throughput(actual data transmitted) seldom exceeds 100Mbps, 100Mbps port is good enough for 300Mbps wireless product.