SSHFS – Use arcfour Cipher with Google Cloud

SSH will validate the server based on the signature of the public key used (a simple hash). It's up to the user to make sure that signature is valid (i.e. you usually need a secure channel for that also, in practice, clients usually just remember the first signature sent by the server and simply warn you if that signature changed).

This means that validating the server (correctly or incorrectly) isn't going to be influenced by your selection of symmetrical encryption algorythm.

That being said, I can more or less guarantee you that changing the symmetrical encryption from AES to RC4 isn't going to yield any noticeable performance improvement: even with a very slow (or starved) processor, the difference in speed between the two isn't going to be noticeable.

if you need more information, someone actually performed a detailed analysis if the performaces of AES vs RC4 (including several mode of operation for AES)

You miss few points in your question:

• What is your openssh version? It can differ a bit over the versions.
• ServerKeyBits is option for protocol version 1, which you have hopefully disabled!

Supported Ciphers, MACs and KexAlgorithms are always available in manual and this doesn't have anything in common with key lengths.

Enabled Chiphers, MACs and KexAlgorithms are the ones that are offered using connection as you point out. But they can be gained also in other ways, for example using sshd -T | grep "\(ciphers\|macs\|kexalgorithms\)"

To get the key length of your server key(s), you can use ssh-keygen: ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub

but you will probably want also the moduli sizes that are offered and used during the key exchange, but it really depends on the key exchange method, but it should be also readable from debug output ssh -vvv host.