The sort of rib eye you describe is commonly sold in Australia. Most of our readily available steaks have little to no marbling. Lean, lean, lean. Nonetheless, I've had pleasing results cooking such steaks sous vide.
In your situation, saute in butter or cook sous vide. If you've gently cooked the steak in a water bath the last thing you want to do is allow the internal temperature to creep up beyond whatever your original target temperature was. And it's not like you want to drop a knob of butter into a screaming hot pan. Use neutral oil with a high smoke point--or no oil, if you've got a non-stick pan--when searing a steak cooked sous vide.
If you want the buttery thing, tho', you could always melt some butter (maybe a compound butter of some description) and brush small quantities of this on your sliced steak.
I cooked the steak tonight. Sous vide 57 deg C for two hours, ice bath for about the same time, then finished in a hot pan with butter. Served with béarnaise sauce.
There was a pleasant crust, then about 1/16 of an inch of gray matter, and a pink interior. The steak was flavorful and very tender -- important for those of us lacking teeth. But it was dry. No amount of butter nor béarnaise could fix that.
The last sous vide Australian steak I cooked had an unpleasantly burned exterior, with a bright pink (perfectly pasteurized but what I would call raw) center. It was not particularly tender.