If you take his extra yard of pace away, I think he would still be a monster. One of the best headers in the game, with an insane leap. Perfectly two footed with a great shooting technique, there are not three players around who have a better goalscoring instincts, his timing in front of the goal will always be immaculate. He's very strong, has good height and his first touch, dribbling and timing would still be as good as it gets.
I remember reading some scientific article on the mystery of Robben, his trademark move is to always cut inside to his left foot. They studied why players couldn't stop Robben from doing that, and it turned out that him cutting inside simply goes too fast. Apparently he can cut inside so fast that it literally happens before you can notice it with your eyes. Within milliseconds he goes to his left, so even if a defender knows what Robben's going to do, it's physically impossible to act on it before it has happened already. Robben's raw pace hasn't got much to do with his ability to cut inside (think of Walcott who has the same pace but not the same dribbling/cutting inside ability Robben has) and if Robben would lose some pace, he'd still be able to cut inside like discribed, so fast that no one could stop it.
Same would apply to Ronaldo, even without the extra pace he would still cut to his right and shoot before people know what happened. You imagine him finally changing his game to become more of a classic striker. He doesn't prefer to play with his back to the goal, and he doesn't need to as long as he's got pace, but he could change it up without problems if his body forces him I think. There wouldn't be many strikers out there who'd have the same amount of quality as an older Cristiano would have, I really don't see why losing some pace would be a problem for him. Chronic injuries or permanent damage would be a much bigger concern for me, with the ammount of kicks he gets and the way he has pushed his body to the limit for years.
But for me there's no way he won't be one of the best players in the world for the next four years if he stays fit, so if we could somehow manage to bring him in it would be brilliant.