Yes, it most certainly will make you slower if you want to race and place as well as you can. Go to a track meet and watch the athletes on a flat surface at a highly competitive level and you will be in no doubt of this. In particular, train alongside some sprinters occasionally - they tend to have entirely different form to distance runners.
However, not over-striding will ensure that you can put in the miles in training, which will help your endurance. Being injured will certainly slow you down!
So, it's a fine balancing act and depends on how competitive you want your running to become.