Is sshfs+encfs stable

Depending on the reliability of your password (how secure it is), it should be effectively unbreakable. It uses high grade encryption from everything I could find.

Note that storing the password on the system you are running encfs on will make it useless. That is, if you store the password in a file on the system itself, it won't take long for an attacker to find it and use it.

If you must store it somewhere, try a USB flash drive, or type it.

You're getting about 80 Mb/s out of your 100 Mb/s link. That's pretty close to the theoretical limit of the link (you'll rarely get anything like the nominal link speed) and is an entirely reasonable figure to achieve.

Beyond that, on top of the overheads of running an encrypted tunnel (SSH) you've also got the TCP overheads, and any other traffic using the connection. I'd say that practically speaking you're unlikely to achieve any better throughput without changing to a protocol with a lower overhead.