The problem with pc-task is that the timing is all sorts of wrong.
Even on my amithlon box (which reports to be running 68040@4800 mips (seems pretty close to actual speed)) the timing goes crazy, even when assigning 256Meg to the translation cache/buffer.
It's nice and fast, but it's inconsistent. Two seconds at about 100 fps for ms-dos quake, then 1 second at about 3 fps. Probably hasn't been noticed by many people as there's very few machines running 68k code as fast as my amithlon box (c2d@4.05ghz).
Win9x does work, but you're limited to vesa.
All in all it's probably the best possible option for 68k systems when it comes to pc emulation, but don't expect it to be a great option, regardless of speed for some tasks.
Considerably faster than Dosbox, but not as consistant, nor does it emulate particular hardware as Dosbox does (gfx is a simple vesa frame buffer in pc-task). Neither is ideal, but between the two you can get a pretty good experience if you have enough grunt.
As for ADoom on the a600, that's not too bad at all. Roughly half of what I used to get on my 1st Pentium system, which isn't bad for an a600.