I bought a Warmoth Jazz neck from their shop stock (so not a custom build). Not quite rosewood, but Pau Ferro fingerboard & the steel stiffening rods.
It is on the chunky side for a Jazz, but I like it as I usually play a Precision, and they do offer a slim option if necessary.
I haven't really noticed the weight of the rods, but I do have Ultralite tuners on it, so that probably offsets the weight.
Remember they do the graphite rod option too, much lighter but more money.
I must say that it's a lovely neck! Plays beautifully.
Si