Are these constants effectively saving computations?

[cblake]

Probably optimized to the same, but it depends on the compiler. E.g., 
TinyCC/tcc would probably not. gcc -O3 almost certainly would. Maybe Javascript 
engines would only optimize it so after 10 calls and some hotspot metric got 
hit. How "performance safe" you should be in such things likely depends upon 
your presumed deployment.