Five months is normal for a Fibracell that is played extensively. I think you can lightly touch one up on the top only. The bottom has a membrane on it that you should not disturb. There shouldn't be a problem on the bottom anyway since it should not warp like cane. I haven't seen the variability in them you mention. I was using them on alto, soprano and baritone at one time. Now I use a Fibereed Hemp Medium on bari. I'm only practicing on soprano now because of band situations so I'm using cane on it which works better - I don't know exactly how I'll work it if/when I get in a situation that calls for soprano. I need synthetics because my 'side horns' tend to sit a lot and cane would dry out - I need them to be ready to go when I grab them. I'm still using the Fibracell on alto but I'm not quite happy with it, so I guess I'm looking for another solution.
Generally speaking, I've seen the Fibracell tend to be a little 'deader' and the Fibereed Hemp maybe a little too far in the 'buzzy' direction. However, depending on the mouthpiece, the Hemp can be very good. For some reason it works on my bari mouthpiece which is a 1975 Level Air 9*.
I also have a Fibereed 'Carbon Onyx' tenor that I haven't tried yet. My tenor piece is a Guardala 'King Curtis' (.116) so if its a buzzy reed, its going to be too bright. Only one way to find out, so I'll post when I know.