Adding weight, but don't want a big wheelie....

If you don't have a passenger seat, ad a nice heavy one. You could also strap some ballast underneath it.
I had two cars I wanted to slow down, one to get from high nines to low 10s, (no license, and I didn't want all the b's that goes with it.) Disconnecting the secondary slowed that car down a couple tenths.
The other car was about three tenths too fast for the bracket I wanted to run in, so I built a permanent stop, kinda like the moroso one above. That thing will be consistent, and slow you down as much as you want. Your problem is going to be lack of tnt to set it.
Isn't it a shame that it is so much easier to slow down than speed up?