The "dream machine" would be to swap the 65CE02 with the 65C02 in the //c+ and overclock the system to 10 MHz.
Might be possible - at least as long as the IIc+ cache runs along.
Even just running the 65CE02 in an Apple //e at the stock 1 MHz speed would seem to provide some acceleration, because the 65CE02 is an improved CPU in a number of ways.
Beside new instructions (which are less fantastic than they sound - not to mention the possible problems added by the Z register), the most important improvement might be the saved 'dead' cycles on single byte opcodes. Since these are also among the most used (e.g. INC/DEC X/Y), an over all speed up of 10-15% is not impossible. Sounds great, but then again, anything below 50% is usually not really visible in everyday life.
Also, the 65SC02 will do the same trick :)
Is such a CPU swap for any Apple //e, //c, or //c+ possible and worthwhile?
Yes and no. Except for
STP the instruction set is exactly the same and unless there are timing specific routines, it should work as well. Thus a IIc+ with it's 800k drive will work fine. Classic DISK II drives may not.
And as said before, the over all gain just from the CPU is not really worth it from an objective POV. Not withstanding of course the pleasure from doing so (I did the same back in the early 80s by plugging a 65SC02 into my II+. I always had that good feeling of having an enormous fast machine - without any proof :))
Raising the clock may gain more - visibly more. Then again, as the higher clock speed on the IIc is only partial (due being a cached system atop a basic 1 MHz design), going past 4 MHz won't be as effective than that step already brought. At least every write operation will slow down (and synchronize) the faster CPU with the 1 MHz backbone.
So, yeah, go for it and don't forget to tell the results :)