Not just excessive but pointless. Since salt will have 4 bytes appended to its end and then hashed using SHA512 then having anything bigger than 60 bytes is completely pointless.

In other words whether you are using 1 byte passphrase (assuming each char is 1 byte) or 20000 bytes they are both hashed to the same 64 byte result and the attacker could simply skip attacking the passphrase and move to the next step meaning they don't need to check a byte[20000] as their input, they simply skip the initial HMACSHA512 in PBKDF2 and check the byte[64] instead.