You really really really need a spoiler, check out this video.
In short, the Evo Spins when he lets off the throttle with no spoiler. They put the spoiler on the car as it works, not because it looks stupid, and yours is much better...
http://uk.youtube.com/watch?v=-0oJ2wQiu2Y
A small video of my car spining at the MLR 30-130 day at RAF Marham 03/11/07 - previous run of 182mph went fine, second run i removed the rear spoiler as it was bending alot at high speed; i hoped it would give me another 10mph, instead it gave me zero down force and zero stability once i came off the gas! Took a deep breath and held on until the ride finished!
Some stats from the V-Box data logging:
I lifted at 183mph and it only took 5mph for it to become unstable and went into a spin at 168 mph.
Maximum spin angle was 142.55 degrees at 114mph before starting to spin back the other way.
The spin lasted over over 330metres