In that you won't have 16Mb of 'system RAM'.
The SCPU will quite happily work with the REU too, so you can have both, rather than just speeding up vice and only having REU-style RAM.
One practicle use for this is:
You can run Wheels in the REU RAM, freeing up the system RAM for something like geoZip, or TheWave.
In terms of speed, well warp is just that, whereas the SuperCPU will be clocked at 20Mhz, but you can still realistically use the keyboard and so forth.
Plus, a RAMDisk created in the system RAM is very fast ;-) and handy for unarchiving .d64/.d81 images. This will mean no disk errors on the transfer, then you man copy these files across to whichever device that you like.
Also, I don't think GEOS supports >2Mb of REU RAM withour patching, whereas Wheels can use 16Mb of SuperRAM (still needs patching to use greater than 2Mb of REU/DMA RAM).