If Hardened pipes is active, I just change the recipe and use different locale strings for the turret to make clear that this is the de luxe version. So there's no upgrading involved, and if you should ever deinstall Hardened pipes, the "Extra strong Fire extinguisher turrets" will be silently downgraded to normal "Fire extinguisher turrets", without the additional resistances.
There's no way to make "Fire extinguisher fluid" yet. I've stolen a definition for one of our fluids in BI for testing, because the original plan was to make the extinguisher turrets use this exclusively. But I didn't come up with a plausible recipe, so I let the turrets use water as well. You could use the extinguisher fluid by cheating it with Creative Mod (that's what I did during testing), but of course that's cheating and not the intended way to use this mod.
I agree, that's not optimal, but I've been working for too long on the mod without releasing anything, and apart from the fluid it seemed to be working. Perhaps you can help me out with an idea for a recipe? It should be possible to produce the fluid with vanilla ingredients, and there should be some chemical stuff in there so it's reasonable that it isn't available right from the start and can't be used in normal water turrets. I'm also open to adding ingredients from other mods, if they are active, but I wouldn't like to add a hard dependency on the full Bob's + Angel's suite. :-)