And output_weight was effectively -3 in the old code, it was just done in a different place.  Take a look at python/spa2/thalamus.py in Nengo 1.4, in the connect() function (the parameter is weight_GPi).  

That said, this was a pretty arbitrary parameter.  Still, I've never tested anything without an output_weight of -3, so I'd be pretty cautious about changing it.