Pace can be enough to make someone good turbo, James Roberts is making a living out of speed. I agree at the moment Guthersons is a turnstile in defence. But he too will also put on weight and get slower. The only thing I see different to you. Is I've seen enough skills in the halves and at full back from Gutherson over the years to say he is better then the average player.
These guys that aren't fast, aren't big, arent real tall and still aren't out of place in the NRL. You have to find a place for them, they have something which makes them very very good. They just read the game better then most. And you want the ball in their hands.
His defence will improve.
But sadly his value would have only gone up this year. He won't be back.