Re: MRI scheduled, another fusion in my future.
Most people don't notice any difference because L5 attaches to the sacrum which is fused to begin with. L4-L5 is the segment that carries the majority of spine movement and that is the one you notice the most...but adding on L5-S1 shouldn't make any difference in that regard.
The SI joints can be more of a problem...but is usually one that can be overcome. I had some problems for about 9 months post fusion but with careful PT, the body learns the new limitations and can make an adjustment. I think where people get into trouble is they enter into PT too early, get worked too much too soon and end up with an irritated sciatic nerve, etc.