I was good making 290 on stock C1 sleeves and no block guard. As raiden said, if done right, I suppose they are helpful and a cheaper solution to real sleeves, but you just have to get it done right. I was pushing something like 12/13 psi on a T3/T04E, but my tuner was always warning me about 'putting a nail in the coffin' every time I went to the track on that pressure without sleeves. Just some things to consider.