With the E8X platform being as light (relatively speaking) as it is, the N52 may be exactly what you are looking for. The E9X platform is a few hundred# heavier, which will make the N55 feel less powerful.
Now, if you do purchase the 128 and feel its under powered, there isn't a whole lot you can do. If you get the 135 and feel even that is under powered, you can get a light piggyback tune and have more power cheap and instantly without getting your hands dirty.
For a 4k difference, I would take the 135. And even then, I would consider a CPO '10 instead for value as well as the N54.