The high temperature will cause some power loss, but not much in large installations. An other competing battery technology is sodium - sulfur also needs to be kept hot to work, though not that high in temperature. So this technology will not work for small units, but large units (e.g. 10 tons and up).
Using lead is not a problem, as for these large installations recycling is usually not a problem. Something like 10 t of lead that such a battery may use have a value significant value. So they will not forget to recycle. So it would not even be a problem if the would use more toxic materials like mercury or cadmium. The bismuth may also be the more problematic part than lead.
Sealing the battery is needed to keep water and oxygen out. So you have to protect is from getting wet anyway - water will evaporate before even coming in. As such a system stores a large amount of energy, there is always a chance for a strong reaction is case of a breakdown - but there is no way around that if you store energy. If the battery should leak, the heat insulation is one of the first things to break down. So leaking lead or salt will solidify - leaking sodium / lithium might burn.