If Notice + Alertness is a problem, I'd make sure you're applying all possible penalties to the roll. Opponents who don't want to be seen are going to use cover, concealment, camouflage, darkness, and any other possible means to avoid detection. All of those will make the Notice rolls more difficult, which will more than offset the +2 from the Alert Edge. Against any halfway competent opponent, the TN should be much higher than normal.

Speaking of competent opponents, if they have the Stealth skill, let them use it to oppose the Notice roll. A wildcard enemy with a good Stealth roll (and a benny to spend) can be extremely hard to spot!

I'd try all of the above before changing a PCs' Edges, and see how it goes.