On the other hand - a lot of code seems to be already in place, and it's
   supposed to work pretty much the same as on sparc32, isn't it?


not really -- sparcv8 & sparcv9 kernel-level things are really quite
different.  amazingly so.  compare to amd64 - you can run x86 oses
on this directly without any problem.  sparc64 _needs_ a sparc64
kernel whether it is compiled 32 or 64 bit.

chuq could probably explain better than i can ;-)