Has anyone lowered a Demon and had any problems and what springs did you use and how much did they lower the car
Agreed. You will ruin how the suspension is supposed to work.
The factory put lots of RD into getting the IRS suspension to work as good as it does and it does work well for what it is so I would not screw it up if you want to race 1/4 mile.
If you want a road car then go with a Redeye, which I see you have and lower that if you want a better handling vehicle.