Ofc make your own benchmarks before taking PLV8 into consideration.

In my case PL/pgSQL FOR loop was particularly slow; lack of builtin data structures like map or stack (though stack is emulated in V8 via array) caused performance problems – array operations are really slow in PL/pgSQL (compared to V8), instead of using map I've use indexed temporary table (creation of such is quite costly).

Still you might have a point here – PL/pgSQL stores execution plans with the function, so there is a chance of being more efficient here.