There are other C/Ku LNBFs around. But either way you get what you for.
No LNBF will make up for an undersized antenna. I don't use an LNBF, I use a dual band feedhorn with separate LNBs for C and Ku on a Prime focus 7.5' mesh. There indeed are some signals that are too weak to receive on that. 10' or more would be more ideal, but it's what I have available... Mostly higher order modulation that I am not particularly interested in, and the lower, near the horizon sats (mostly due to local obstruction) will have trouble. I am guessing that an undersized offset antenna will produce spotty unpredictable results on CBAND.
Below are details from my antenna:
Perfect 10 7.5' mesh Chapparal C/Ku Co-Rotor feed, Norsat 8115 CBAND, Norsat 4106A Ku Thomson Saginaw Actuator, Vbox positioner, Home Brew Polarotor683 Servo circuit.
DB4E/VHF Yagi rotor FM Bandstop ap-8700 preamp 4way split LG lcd.