Re: Having additional fusion
The common word on this is that it won't be noticeable to you. L5-S1 has very little movement to begin with as it attaches to the sacrum, which is a solid block with no movement.
Where you may notice some difference post surgery is in the SI joints. This becomes the first level that has movement after L5-S1 is fused. I know many people complain of some problems in this area post fusion. I know in my own case, it took about a year for my body to get used to this new situation, for the muscles to become accustomed to supporting the spine in a new way, etc.
(I was fused at L4-L5 and then L3-L4 and L5-S1 were added on.)