Suggestion is simple. If a character takes either sf: spot/listen/search or esf:spot/listen/search they also gain the keen senses feat if they didn't already have it. For those that already possess the keen senses feat, there's no added benefit.
Keen senses has some pretty profound uses in combat in conjunction with stealth and it seems odd to permit only elves and rogues from assessing a feat that cripples spot investors that are not these classes/races.