Hi Keith,
I love love love the Gotoh 510s, especially in antique gold. I not only upgraded past Taylors with them, but my present ones all have them. No, nothing wrong whatsoever with the stock machines; they work perfectly fine and have never had issue with them. The 510s are simply superior in feel and precision and are "worth" the expense for me.
That said, the factory charges a
hefty premium for machines you can simply (and I do mean simply) fit onto the guitar yourself. So whether you pay the piper and check off that box, or DIY is entirely up to you.
The Bone nut/saddle
is a good upgrade and worth doing, IMHO. Again, nothing wrong with the stock tusq/micarta. But bone on both nut and saddle is a good choice, not to mention Taylor's "wave" bone saddle is a lovely piece to look at (no joke, it's a beautifully-shaped piece and satisfies that mechanics nerd in me
). Sonic difference, if any at all, is almost impossible to predict. But "generally" (whoa, lots of grains of salt there!), bone is considered the superior tonal material. I'd do it for the "potential" of deeper, clearer tone, but even if no sonic difference, I do like the look of it, especially on a higher-end guitars as if it somehow "rounds out" the entire package. ...oh so subjective
Edward