If Surfer were to increase his transmutation powers he would move toward reality warping. Superman doesn't have that. Surfer has telekinesis, how powerful would he be in his area with a boost? He's altered planetary atmospheres and healed others and he has limited cosmic awareness. Boosting all his powers would make him exponentially more powerful overall. For Superman, it means marginal increases in a character that is super strong and super fast.
I don't think the Gem would affect exotic abilities as these don't require more power to be more effective. It would affect blasts, strength, and durability.
I'm confused at why Superman wouldn't need the power gem? He's not the most powerful person in DC. I've heard a variety of stories about Nu Superman getting his ass kicked, so how the hell couldn't he use a power boost?
Also from the way they describe the gem it implies you could use it's energy to essentially give yourself any super power you want if you became skilled enough with it. But even if it just boosted the powers you already have...how could he not need that?
__________________ Chicken Boo, what's the matter with you? You don't act like the other chickens do. You wear a disguise to look like human guys, but you're not a man you're a Chicken Boo.