Integrated Sachs Wolfe Effect - redshifting?

[ChrisVer]

I think this have been discussed a lot, however, here it comes again right in front of me. I'm talking about this article (and also in my class notes):

http://en.wikipedia.org/wiki/Sachs–Wolfe_effect#Integrated_Sachs.E2.80.93Wolfe_effect

and in particular at this paragraph:

Accelerated expansion due to dark energy causes even strong large-scale potential wells (superclusters) and hills (voids) to decay over the time it takes a photon to travel through them. A photon gets a kick of energy going into a potential well (a supercluster), and it keeps some of that energy after it exits, after the well has been stretched out and shallowed. Similarly, a photon has to expend energy entering a supervoid, but will not get all of it back upon exiting the slightly squashed potential hill.

So my problem is, how can you talk about an accelerated universe when you look at a supercluster object?
The accelerated universe makes sense in the FRW metric. However a supercluster (a group of galaxies+clusters that I believe are gravitationally bound -and so the gravitational redshifting of ISW) cannot be described by FRW metric... So how does the well gets "stretched" during the photon's stay in it?

 

[Chalnoth]

A cosmological constant makes it so that gravitationally-bound systems aren't perfectly stable over time. The larger the system is, the more it is impacted. The cosmological constant causes gravitational potential wells and voids to decay over time.