Would the
open feedline method reduce 75M performance compared to the regular
coax
fed NVIS dipole?

Yes, slightly. But not enough to really worry about if you are
careful tuning, and use the least inductance setting. But yes,
unless the coax fed is hundreds of feet long, or thin junky
coax, usually it's the least overall loss on HF. I use fanned out
dipoles for
separate bands fed with one coax. MK