Cmon, dude, the stock engine cover is a solid piece and the NSX-R is mesh. Which one do you think will have better sound insulation?
I don't think the stock engine cover provides a noticeable level of additional quietness compared to the nsxr engine cover
If you go plexiglass you would notice the different look a lot more then any difference in noise, but I believe you will never top the look of the oem nsxr engine cover