Many carnivals/county fairs have a booth where novelties are made by heating beer and soda bottles - often there's a sorta collar with electric heating coils that fits around the neck of the bottle.
You may have seen these stretched-out bottles.
I don't know what the loss rate is.
There may be tricks I did not notice before i started working glass - such as heating the bottles up in an annealer, and then returning them to cool down slowly. All I remember is the wow factor of watching the bottle sag.
I can also tell you that glass for bottles is formulated to quickly solidify - to suit high-speed molding. That may mean that it can better withstand sharp changes in temperature.
That may help your success rate - but I would still take the step of annealing my work before I sold it.
One other consideration - there WILL be minor variations in the melting points of the scrap glass you are using. That means some shards could soften and flow at the annealing temperature of the bottle. This could actually yield some cool effects.
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