Balance your outlet with whatever the intake is. Don't get too little of one and too much of the other. The best airflow will occur when your intake and outlet CFM are balanced. Too much of one or the other and the air won't flow right.
The 5 1/4" bay fan is adjustable and can do 50-100 CFM. Most of the time, you will probably just leave it on max. Assuming that, you will want your total outlet to match (give or take some) and total about 100 CFM. Depending on what your current outlet fan is capable of. If it's a generic or standard model, it's probably not doing more than about 50CFM. Some higher end ones will do up to 100+. So depending on what you have in there at the moment, you may only need the intake fan. I'll bet that by just adding the front one, you'll see a difference in temperatures.