Sounds like timing is an issue. Do you work with anyone?
If you are just accurate with the slide step, you are losing velocity. I suspect that the reason you feel like you throw harder this way is that you have your timing more in sync with this particular profile. It would be best for you and everyone to see you on video. I prefer game situations..though they have more of an issue with angle and what not...it shows what you do when the rubber hits the road. Any video could prove helpful though. Do you know any drills that help you work on timing? If so while waiting for video, I'd suggest you find and do some timing oriented drill work and see if it doesn;t improve your issue..I'll go ahead and delete the multple postings.