I went ahead and did some in-executable benchmarking, I don't have access to advanced profiling tools or the time/will to figure out the free stuff at this moment.  

From my tests, there's basically no difference (+/- < .05%) for lines shorter than 5 tiles or so, it *looks* like there is a ~.08% improvement for lines over 10 tiles. I only tested up to 20-tile lines.  I didn't take the time to reserve a core or anything so I wouldn't say this is in any way definitive.  

Interestingly enough, it looks like the "new" 3D lines_to is something like 8-9% faster than the old function when dz is 0, at least on my machine. No idea why that is, but I'm getting ~950000 calls completed in 10 seconds, rather than ~860000 for either version of the 2D function. Maybe it's my compiler, maybe it's that the tripoint struct is in contiguous memory, maybe it's something else entirely. Either way, that seems more significant.