Be careful as there are two distincts problem.
Then, in addition to that there is the "meltdown" attack. This uses largely the same idea, but because of an additional problem in intel (and, so far, only intel) CPUs, it can also be used from an userland application to directly access kernel data. This is the one that led to that large patch in Linux to unmap the kernel memory completely (not just read/write protect it) when userland code is running (which is a little costly - but newer Intel CPUs already provide a faster way to do this).
And yes, if we were to fix this we would probably implement a "4G/4G split" for 32-bit Haiku. Since the kernel and userland memory do not need to live side by side in memory anymore, we could as well have each of them allowed to use the full address space, which could help with porting Wine and would also allow apps to use more than 2GB of memory, a limitation we sometimes hit (for example if you open a lot of big/complex pages in Web+)