Another project: http://wanproxy.org
But still this space is heavily owned by big corps. I guess smart developers/startup could find some real market here.
The network infrastructure needs to be configured to divert traffic to the boxes which is fiddly and becomes a support load very quickly. If SSL forms part of the traffic you want to accelerate then there are two approaches either of which require even more system administration (not network administration).
Once a customer is non-trivial (eg 3 or more offices) then suddenly the sales process slows down drastically as the same product has to be used everywhere and supported by the networking and software people.
What this means is that if you are competing conventionally then it is very hard against the now entrenched vendors. If you are trying to be open source then you will find a very long hard and extremely likely to be an unrewarding slog. Many of the small vendors were out proving the space when the big corps all realised they needed an offering, and did all the acquisitions. The changes of being acquired are very slim now.
What do you think about Silver Peak virtual WANOP solution ?
No idea. However they aren't a small startup/open source solution like you were originally looking for! Pretty much everyone's solution is written as software that runs on Linux (sometimes BSD). That then gets packaged up on an hardware appliance, suitably tested, marked up and sold.
Consequently not much is needed to make a virtual appliance, although testing is harder as configurations are more diverse. In other words it is likely to be substantially in behaviour to hardware appliances (unless they deliberately exclude things to protect hardware appliance sales).