Security of bitcoin private keys are 128 bits since the security of Elliptic Curve keys are defined as half the key size (bitcoin curve is 256 bit). On the other hand security of entropy (if generated randomly and is strong) is the same as its size. If the 600 words are selected using the same word list as BIP39 with 2048 words then their security is 600*11=6,600 bits. A pointless increase but an increase nonetheless.