Once they grow to the top of our trellis (3.5m), I train the them sideways, you have to keep wrapping them down otherwise they try and head up, but they can be trained easily enough. With a few extra horizontal wires, you can train some of the laterals as well, which produce a lot of flowers.
Not sure about yield reduction for a low trellis though, smaller plant smaller yield is probably right. Just expect a healthy vigourous plant to create loads of vine and leaf, which has to be trained somewhere, or cut back.
I read somewhere that side shooting, or the top vines falling over sideways is a signal for the plant to start creating flowers, time of season probably plays a part as well, by the time they are falling over its probably time to flower anyway.
Another interesting thing was a commercial grower on a utube video (yep theres a few, check em out) saying that the vines do their growing during the day, and wrap themselves up at night.