Raul, I'm sure the OP was referring to 30% packet loss with FEC, which opus can turn back into a usable audio stream. You'll absolutely notice artifacts in the audio, and you'd clearly identify something is wrong, but it's a heck of a lot better than 30% packet loss without FEC.
Colton,
Freeswitch supports opus, yes. I'd be curious to know why you want to transcode between opus and G.711, unless your endpoints don't support G.711 though. Yes, FEC can help you with packet loss on one end, but if your packet loss is on the G.711 end, then you're SOL anyway, so it seems pointless to do this unless you can use opus all the way though.
As to how it will sound if doing the conversion, well, you're transcoding from one lossy codec to another lossy codec. The end result will be audio that will not sound as good as if it were just encoded in G.711 to begin with. Remember, the nature of lossy codecs is that they're, well, lossy. You can't make a G.711 stream sound better by re-encoding it to opus. Once it's encoded in G.711, that information is lost.
As for CPU usage, opus is not particularly cheap. Transcoding + resampling (if required), will considerably limit the number of concurrent calls your FS instance can handle. Actual results are going to depend on a number of factors that make it infeasible to talk about it. But again, you probably don't want to be transcoding between these two codecs unless you absolutely have two (i.e. you can't get the two endpoints to otherwise agree on a codec). You're not going to gain any real advantage from doing this.
Best,
Colin