Actually there are two schools of thought on that.
It might actually increase overall ambient temps in the case. Main reason being is that it introduces turbulence that can interrupt the airflow which runs from the front and out the back.
However, CPU and GPU temps will probably benefit from the direct airflow, but other components can run warmer.
The other consideration to take into account, if it's an issue, is the added noise. Ended up going back to stock airflow, unless you're adding a 50+CFM fan, the decrease in temps isn't worth the added noise.
I've modded 15 or so cases in the last 3 years. Let's just say I went out and bought 60MM, 80MM, 92MM, 120MM hole saws and made sure I used all of them more than once.
P4 2.6C, Thermalright SP94/Panaflo 92MM M1A, IC7 Max3, Swifty MCX159, 1GB Geil PC4000 Ultra Plat, Radeon 8500, Audigy 2, 2X WD Raptor RAID 0, WD 250GB SE, Plex 708A, 166SDVD, LianLi PC75, XP Pro.