I typically use 8 - 10 lb fluorocarbon when targeting the coho. However, lately I've been having difficulting landing the fish as I'm finding the 8 lb fluro breaks easily when the fish is close to shore. Might be time to go back to 8 lb Maxima ultragreen if the fish aren't too spooked. And yes, IMHO - coho can be very shy so every advantage helps - incl. going down in leader size in clear water conditions. I've even gone down to 6 lb. flurocarbon and hooked coho when the 8 lb isn't working.
The advantage of flurocarbon is stealth. However, a big disadvantage is a lack of tensile strength compared to regular mono like Maxima ultragreen, which has more strech or "give" and the fish won't break off as easily.
As for springs - well, I've landed them on 8 lb flurocarbon, but it depends on if the fish "cooperates". I do find that once a spring is subdued, it gives up fairly easily. Coho, on the other hand, never give up and will often give you a final fight AFTER the fish has been landed. I've seen, and experienced, many times when the coho is "landed" only to get away when an inexperienced fisher tries to tail the fish.
Btw - ericd, were you using flurocarbon or regular mono for leader?