improvement is a relative thing. haf is notorious in having little to no air filter so it's engineered to be a dust magnet. if you cut down your airflow by adding air filters, you should consider adding more intake, OR reducing exhaust. IMO taping off the top rear exhaust fan will actually help in the overall airflow pattern of the case. this way you only have 1 exhaust on top (draws heat from the RAM sticks) and 1 exhaust rear (your H60 rad which also draws heat from your VRM'S) this SHOULD make your case more balanced and improve overall thermal efficiency. you won't see a drop in temp, but you will see a drop in overall sound and dust level.
if you are REALLY fixated in getting more cooling, you can buy a higher quality 200mm fan to be mounted in the front replacing the 2x120mm fans you have currently there. switch the side panel fan out to a high quality 140mm fan instead, and maybe add a 3x5.25 bay fan cage like this
for additional cooling
however. IMO that is way overkill for your system. you wouldn't be using a HAF 912 if you have THAT much heat being produced in the system
most likely your existing thermal solution is perfectly fine and you are just nit picking trying to justify the $ you spent on fans you didn't actually need