A:
Size is only compatibility problem I've had with flash cards. Some devices will not read / write, past their size limit. Some will not read at all.
Your device might have speed limit, but would still work, in my experience. Like putting race engine in John Deere tractor ;-)
Sony Cyber Shot isn't specific. Plus I don't own one. Your computer might benefit, even if camera does not. You'll likely buy quicker gear as standards improve.
I'd rather buy affordable top performance, that will last longer.