I do have Bonzai but not using it in my workflow since there is not yet the capability to export 2d axonometirc hidden line geometry which I need for my architectural construction documentation. Once they implement this I will try it on a project and see how well I like working with it.
There is currently not a Maxwell plugin but I remember that it was mentioned that this is forthcoming. Currently there is not photorealistic rendering inside Bonzai, but you could always export an obj file and import into Studio for the time being. They have said that photorealistic rendering is coming soon, and I'm assuming more than Maxwell...but that's an assummption...they (Autodessys) does not say much about things coming
The stability issue is a concern of mine....the beta versions crashed so much that I stopped working with them but I have not worked enough on the released version to tell how much of an issue it still is...if you go to the bonzai forum you'll see that there still are some stability issues. They just released v1.1 which reportedly addresses a number of crashing issues.
The "speed issue" is something I'm going to look at carefully as well...I too had used SU some and found it came to a crawl very quickly with larger scenes...a lot has to do with how easily objects/layers can be turned on/off...but when it comes time to set up scenes for renders then obviously the program has to be able all the geometry you've thrown at it. But sorry, for now I only share the question/concern....but no answer. I might say though that I did import a sizable formz model into bonzai and it was no problem at all.