In the standard GW lists, you only run 4x StP main and 2-3 PtE in the board.
With a pFires list, you get to add 3+ pFires to your main which gives you a considerably higher density of removal especially considering you can recur it with Groves (even doing it once before it gets Wastelanded is very powerful).
With the GW list, I have too many times opened a solid hand with no removal and then my opponent drops a Mom T1. Even opening 1 removal spell isn't enough many times since they drop Mom and you have to deal with it and have nothing left for the followup Knight.
I'm not entirely sold on Ledgewalker as some sort of mirror breaker. Maverick runs plenty of answers to equipment and once you deal with their equipment, Ledgewalker gets really bad.
Things you say for GW to do against pFires (getting Thrun, Ledgewalker, Ooze, use Wasteland) are things that the pFires lists can do as well. However, on top of that, pFires lists get to beat active Moms in the mirror as well as keep removal back for your Knights whereas GW builds have to spend important removal on Moms and have a pretty tough time getting through an active Mom at all.