I haven't called my "real" plumber yet but the GC's plumber is telling
me that someone will have to rip out the back of my cabinet, cut into
the plaster and concrete block, just to lower the drain pipe.

I've seen this before, it's a common problem in kitchen remodels, so
much so that my local building department makes a point of mentioning
it whenever a permit is pulled for any sort of kitchen remodeling work
- if the outlet from the disposer is more than an inch or two low he's
probably correct, if it's less you may be able to find a disposer with
a slightly higher outlet.

If you do decide to change the plumbing, make sure you end up with a
seperate clean out accessable without removing the disposal.

Michael Thomas
Paragon Home Inspection, LLC
Chicago, IL
mdtATparagoninspectsDOTcom
eight47-475-5668