一段、一層 are already explained in this text a little bit. Now, let me use them in the sentence of よいパソコン In the text. I suppose that using ぐっといいパソコン、めっきりいいパソコン to shop clerk is abstract.They might wonder how much better PCs you want to have a look. On the other hand, 一層いいパソコン、一段いいパソコン、尚更いいパソコン are more concrete and it implies that you want to have a look at PCs which is better than you are checking now by one rank better or couple of ranks better PCs. So, it sounds more specific.