manofsanATyahoo.com wrote:
Recently there have been announcements about new and improved ion
thrusters.

One of them was the Helicon Double Layer Ion Thruster, and the latest
one is the DS4G Thruster.

Each represents different improvements in ion thruster design. I would
like to ask -- are these respective improvements mutually exclusive, or
can they be combined together in complementary fashion for an even
greater performance boost?

I dunno, they look pretty mutually exclusive to me.
http://www.esa.int/gsp/ACT/propulsion/ultra_ion.htm

One of the advantages with the DS4G is that the
double grid reduces the severe grid erosion inherent
in other ion drive designs.