From the MCC June Developement Update:
Postums: With Halo: Combat Evolved coming after Halo: Reach, a lot of people have been asking about our planned improvements to HCE in order to bring it more in parity to the original Xbox version. Essentially, what we are planning to rework to be more aligned with legacy than the original Gearbox port to PC that is used in MCC.
Michael Fahrny: We’re not targeting it for the launch on PC, but we are continuing to investigate what we can do to bring CE back to parity of the original Xbox version.
But, what I can say, is we have dug up some original Xbox devkits that we are looking into utilizing.
The MCC dev team is aware of the demand. Though it might be a while until CE’s graphics are fixed.