I did this last night and the driving position is much improved!
I do have a couple of concerns though.
I think the plastic parts are there to centralise the stud as it goes through the hole in the floor. So there is a risk that if the seats not central, then the nut on the bottom may not be central and seal correctly. Could solve this by fitting washers on the underside.
Taking 20mm out of the bolted length may not be too good for the bolted joint either, but it's difficult to say if its close to its design limits. Could also be solved by fitting 20mm of washers to the underside.
I think the reason the sliders are now stiff is because your bolting the runner down against the carpet and, in my car at least, over a fold of carpet or overlap in the middle of the centre runner. This means your trying to bend the runner like a banana as you tighten down. Washers between the runner and the floor as others have said would probably solve this.
Thoughts?