you would get a short lanky plant, that would start browning from the bottom up, halfway through flowering, that woldnt yield anything.
6 inch square pots, Veg until 3 nodes, clip the top and flower.Go with a short strand, lowryders or an indica. Still a good chance you run out of space.
The only real reason to flower early, is to sex a mother, by flowering her clipping.
Root bound is to a plant, what a small tank is to a fish. It doesn't cause death, as much as it hampers growth!
Yes, it will affect the plants' size - but this isn't a good way to keep them small over a long period of time. Problems will arise at some point as the plant tries to grow but can't.
"Extream Measures" can be used to offset these problems: Root trimming (Bonsai growing method) is a good way to keep a plant small. "Larger plants" can be grown in "smaller pots", it just takes a certain amount of knowledge and experience (everything happens "faster" this way, including water consumption, nute consumption and onset of problems - so be careful!)