Unless modded cars are still a big problem online, most drag rooms won't need to limit to PP. I've found that, while testing months ago with another tuner and we both came to the same conclusion, downforce of any kind is bad for drag racing - whether it be passive downforce like flat floors/natural car aero or adjustable downforce on front/rear spoilers. While some cars can't remove it all, there are sometimes alternate models with similar horsepower but less natural downforce and will be faster in theory(it isn't always the case).
The only case I could see flat floors being useful is on a circuit track with short straightaways and long, arcing corners where lateral grip is essential for carrying speed. Something like Tsukuba Circuit.
