Basically it only has jumpers for voltage and multiplier — it won't give you support for 100MHz CPUs unless your board supports it. And if your board does have support for 100 MHz FSB, it can likely use those chips without any upgrade kit (just maybe need a BIOS upgrade for plus CPUs, depending on the board).

So AMD K6-2@400 is the highest clocked 66MHz CPU and there's that.

I've heard some people change the chip to K6-2+/3+ just for setmul support and (maybe) more cache. The chip will work at 66 MHz, but you'll be able to set the Spectra to 2V. I must tell you that the chip sits pretty flush on the kit. I haven't found the courage to try and remove mine (not that I need that terribly).

The user Dragon Caesar did that here, though

Yes, there is potential with that voltage adapter. I had done this to use a K6-2+/450 CPU on a Gigabyte motherboard with an Intel 430VX chipset that doesn't support 2.0v-2.1v.

The biggest barriers would be the maximum FSB frequency that the motherboard allows (either by overclocking or official motherboard chipset support) which would limit the maximum CPU frequency and if the BIOS could perform POST successfully (like jheronimus said it may require a BIOS upgrade depending of the motherboard).

You would also need something like a lever to remove the current installed CPU because the socket on the adapter isn't ZIF.