Is there a difference between USDM tailights on a MR vs GSR for the IX?

 

negative

 

i heard somewhere theres a difference on the black in the tailight, something about the MR has a flat black finish versus shiny black for the GSR ...

 

You're getting mixed up with JDM vs. USDM tail.

For JDM: I honestly can't say if there were differences between GSR and MR for the JDMS.
VIII MR Tails are flat black
IX MR tails are glossy black

USDM:
VIII tails are altezza style