Believe it or (k)not, a left handed bowline is considered to be 50% weaker than the "real" thing by a lot of classic references in climbing/caving literature. I believe the current opinion is that a left handed bowline is a tiny bit, maybe 10%, weaker. This is such that if the rope breaks, it is probably going to break somewhere other than at the knot where use or abuse has caused hidden damage. If a modern rope used anywhere near correctly fails, it is almost always due to cotnact with a relatively sharp edge under load