My LG E10N (which I've been using for 4 years without a hitch) is an older model, and aside from using an internal LG drive in an enclosure, is the only external LG I've used - however, I did find the GE20NU11 model online, which is a 20x USB drive.
The other LGs I've seen in the shops myself have all been slimline 8x ones - such as the GP08NU2.
The external Samsung I have is fairly recent, the SE-S084C - but once again, this is an 8x slimline drive. The SE-S084C also gets its power from the host system, so this could be a problem for some (although it works great on my two desktop PCs). Unfortunately it's the most recent external Samsung that I know of. And mine at least is a bit ropey with -R DL.
Another option for you could be to buy an external drive enclosure, and put the internal drive of your choice inside. And you mention fitting an Optiarc - it's certainly an extra option. :iagree:
Perhaps others will chime in with more opinions and recommendations.