I took out the backseat in my prelude and underneath were 2 heavy black metal plates that I also removed (They were bolted to the metal beneath the seat and to the side of the car)

Will removing these possibly affect handling?? The fact that they were so heavy seems to mean that they were there to keep the car stiff like a strut bar kinda um... even though they were underneath the back seat...

Should I put them back on?

 

If they are really heavy, they are used as balancers to balance weight on your car, just my 2 cents

 

I would leave em unless I had upper and lower strut bars all the way around to compensate the body balance. Do you have stock suspension cause if you remove those I would beef up the suspension as well. I stripped my civic and had alot of extra support crap I had to deal with. But I dont know for sure what to say.