Can you implement some features?

1) Load balancing leastconn (like leastconn in haproxy)

2) UDT transport between stunnel instances(with several congestion controls which are already implemented for UDT)

3) SCTP transport between stunnel instances (if can be done in userspace)

P.S. I'm using 400mbit WAN with 55-90ms latency+0-10%packet loss and on one end I can't control TCP settings at all (remote end is on OpenVZ VPS).