Having done the 68030-->68040-->68060 thing I can say that the biggest noticeable difference is from 68030-->68040 (about 6x times faster). Going from a 68040@40 Mhz-->68060@50 Mhz is quite noticeable as well, about 2-3x times faster again. Note the 68040 chips get HOT, so I question the reliability of 15-20 year old CPU's running inside A1200 trapdoors.
The thing with an A1200 though is that it has other bottlenecks: AGA, the IDE interface, but the upgraded CPU WILL also improve things here as well, if you use hacks and patches like FBlit, FText.
My advice if you can afford a 68060, buy it. If not a Warp Engine 68040 @40 Mhz with its SCSI (If its an A3000.A4000,) or an Apollo 1240 or Blizzard 1240 (I think these had recycled 68040's though) will still make you smile, a lot compared to the 68030.
