Soundproofing top of Carbide 500R

Thinking of putting a sheet of soundproofing under the upper grill of my 500R since I don't have fans or radiator mounted there. Case is pretty quiet, but trying to make it even more so short of replacing all the fans.

CPU idles in the mid-high 20s, chipset around 32C. GPU is reference and dumps heat out the back instead of inside the case.

Question is, how much will covering the upper vents affect temps?

Thanks!