The issue with Goombas is that their code is shared as part of a large routine that handles all of sprites 00 through 13 (i.e. Koopas, Goombas, Bomb-ombs, Buzzy Beetles, and Spinies), so there's no simple way to edit their speed without editing all of them. There's also no disassembly of just the Goomba right now so that's not an option either.
Now, one option is to set the Goomba to use the Blue/Yellow Koopas' X speed instead. You can do that via this patch:
db $21 ; don't touch this value
db $0C,$F4 ; the two X speeds it'll use instead (right, left), if you want to mess with them
; note that this will affect blue/yellow koopas as well!
Alternatively, is possible to add an exception for the Goomba's speed using a patch, it's just (sidenote: patch is untested) Professional frame-by-frame time wizard. YouTube - Twitter - SMW Glitch List - SMW Randomizer