The question is not really how big a cam you can run, but what cam would work well with your particular combination. If you have stock heads and exhaust, then an e-cam won't do much for you. If you just want more lift then just get 1.6 roller rockers instead of the cam, that will also free up some horsepower from the decrease in friction. In any case, the e-cam should not make the valves slap the pistons, at least not with 1.5 or 1.6 rocker arms.