@JJ
It feels faster than 680x0 based systems, including 060. Of course, if you have a 040 you will notice your system has become very very fast :-)
Keep in mind that even if your 603e is not the fastest one, the rest of the OS runs native PPC code so even if Petunia couldn't match a 060 in some parts, you probably won't notice it because all the rest is running much faster.
I think Classic release is polished and well finished.
Keeping an old OS3.x for old stuff like WHDLoad is a good idea. I also have one with MOS so I can enjoy all amiga-like flavours :-)
Anyway Xeron has published a list of 68k demos compatible with OS4. In some cases you may get more speed if the demo is more or less system-friendly.
-edit- I didn't have much time to test an A1200 with ZorroIV but I think it will work as Zorro cards were listed correctly. I couldn't test it more, sorry.
I think ZorroIV board is more or less "clean" and probably will work correctly. Anyway keep in mind that some zorro2 card drivers may be more sensible than others to timings so most of cards probably will work but others could have problems.