To be fair, although "erased" data may still be available in the flash memory cells of an SSD, to my knowledge there's no way to get at it short of disassembling the SSD, removing the chips and installing them in a special system designed to dump their data. So if your security needs aren't extreme it may still provide an acceptable level of protection.
Please, someone correct me if I'm wrong.
The biggest impact I can see is that encrypting the volume is going to mean re-writing all of its contents, and that means more write wear on the drive. That shouldn't be a huge issue if you only do it once.
There may also be an impact on TRIM - I don't know enough about these tools to know if its an issue or not, but it's something I'd be sure to investigate.
I don't intend keeping important data on the SSD. The main reason I wish to encrypt it is so I can mount my other encrypted volumes at boot time. I also thought of encrypting the SSD just a little so it can allow me to mount other volumes at boot time.
Thanks for the answers until now. I'll investigate some more. If anyone has anything to add it I would appreciate it.