RAM voltage of 1.65 is most common for DDR3 XMP profiles and is usually needed to get that level of overclocking. I have used several 2400MHz kits with 4770K and 4790K in 24/7 rigs, two of which are still running in daily use. Since yours has also been running with XMP voltage for several years, I'd say that the 1.575v recommendation is quite conservative and may only be needed on particularly weak individual CPUs.
I think 2400MHz is a decent overclock for DDR3, especially for 24/7 use. You could try manually tuning the RAM for higher clock or tighter timing, but I would not recommend any higher RAM voltage. I'm not pressing my luck in these 24/7 rigs and can't recommend it for anyone else.