I couldn't resist the temptation to try a Xeon X3460 (< $13 on Ebay) with my Biostar TH55HD motherboard. The X3460 has two additional turbo multipliers above the X3450 (x25 and x26). I delidded the X3460, applied my own thermal paste between the CPU die and the heat spreader and left my CPU Clock at 160. The only BIOS change needed was to set VCore to +.04V (all speedstep settings remained unchanged). MacOS Mojave needed no changes and CPU multiplier works perfectly (observing low multiplier of x9.0 with turbo multipliers of x21-x26). If you already have an X3450 overclocked as I described earlier, it's not worth the effort, but it was a fun experiment. Attached pix show the delidded X3460, Geekbench results and a HWMonitor screenshot with all 4 cores running at x26 (4.16GHz).
EDIT: Uploaded a new Geekbench result after realizing that my BIOS memory settings were wrong. Newer Geekbench results are with memory timing set by XMP2 (8-8-8-24).
EDIT2: After using the upgraded Mac for a day, the system seems noticeably more responsive. As I noted earlier, the upgrade from an overclocked X3450 to an overclocked X3460 is probably not worth the time, but you won't be disappointed.