okcnaline :
Uh... For Sandy Bridge to Haswell, if you mess with the BCLK it raises the clock of everything (CMOS excepted). It's on Skylake that BCLK only affects the CPU and not other components. That's why Skylake has been getting some noise recently, with the slew of board manufacturers flooding out BIOS updates.
Yes, the BCLK is tied to other things on those chips so you can't change it a lot, but that doesn't mean you can't touch it at all. Read some of my motherboard reviews. Most boards can be stable up to 105 MHz. One Asus model that recently went through my bench was able to hit 108 MHz stable. That doesn't seem like much, but on a 37 multiplier locked chip, that's a 185 - 296 MHz potential boost. It's not a huge amount, but it's something.
I fully understand the Skylake broohaha. Yes, you can possibly take the BCLK up to 120 MHz and above. Yes, this makes things like locked i3 and i5 chips more exciting. Mboard mfrs have been bypassing Intel's lockdown for a while now with H81 and B85 boards that can tweak CPU multipliers when only Z boards are supposed to be able to. These BCLK changes are why Intel is not letting the E3v5 Xeons work on the normal Z, H, and B chipsets. No one would buy a 6700K when you could get an identical Xeon for less and take it above 4 GHz on a BCLK tweak.
Twisted, no, even if two boards are near identical, I still wouldn't try flashing one BIOS onto the other. It's just not worth bricking your board. You saw such dramatic increases in OCing your old CPUs because that helped them overcome their relative inefficiency compared to modern microarchitectures. With a modern CPU, it no longer has that weakness so OCing is often not necessary.
Check this for some detailed bench numbers regarding CPU speed and type.