yes you can reuse it. it's just like hdd.
do you mean to say pcie version 2.0, or x2 (2-lane) ssd?

in general pcie ssd are faster than sata ssd (depending on pcie version and the number of lanes). the minimal in the market now is pcie 2.0 x2, about 750MB/s. for x4 ssd, with ahci it's about 1 GB/s read/write (pcie 2.0); with nvme technology it can reach over 2GB/s (pcie 3.0).

oh this one is plextor m6e pcie 2.0 x2 ssd with aluminum heat sink. the current one in the market doesn't have the heat sink.

this one is my target hehe

pcie 3.0 x4 nvme. speed is over 2GB/s read.

Some of asrock line had such slot, but it will eat into your PCIE usage for GPU though.