I'm not sure if you will see degradation in the picture or not.
For me, a less than optional signal strenth led to audio drop outs and me not being able to use the PVR as the signal would drop out and interrupt the recording.
I remember HD looking HD when the signal was coming through at enough strength, otherwise I had no reception or "blocks" of reception flashing on the screen. I guess the box either gets the signal and shows it on the TV, or it does not and you don't see it. I don't think low signal strength would make HD look like SD.
For me the bigger concerns I detailed above were the reason to have a cable co. rep come in and install a signal boost.
Just an FYI - my low signal strength was not due to splitting.
My "Stuff": Hitachi 51F500, Denon 889, PS3, Paradigm - CC390, Monitor 9's, Atom Monitors, HSU VTF3 MKII sub.