Running the rads seperate from the case will lower temps. You will be running cold air from outside, and you won't be pumping warm air into the case. People do it that way to keep everything together and portable. To avoid that issue, I installed my rad inside my case but ducted it across the two side walls (120mm pushing from starboard and 120mm pulling from port). No air from the rad gets to my system and it is fully self contained. The only issue is lining the fan up with the duct when I put the side back on after tweaking it.
Oh, and if you have any doubts to this, try sitting in front of the rad exhaust on a day that is 95F ambient/30 pct humidity and you're running your system at full speed. You will positively melt. You really don't want that air on the rest of your components if possible. I firmly believe in either FORCEful exhausting or strict ducting if the rad is inside the case. My case interior temps dropped dramatically after I did this.