Note that about all of this is from memory and could be wrong, but here's what I remember:
When we ran CW-Sieve before, it only supported CPU and Nvidia GPUs. We would really like the to restart the subproject with support for AMD GPUs as well. Of course, speed improvements are always nice as well. I believe it's been close to a decade since that software was written so I imagine new CPU and GPU features could provide considerable speedup if the software supports it.
IIRC MTsieve was able to outperform one of the old programs but not the other. If we do use MTsieve (which I think is likely unless the old app outperforms it), we'll also need to do more testing and write/adapt a boinc wrapper to go with it. This is in addition to admins going through the old server side code, as well as doing some initial sieving to prevent initial uploads from being huge.