Thoses at the rally would have noticed that Stillen's car had mods on the front splitter, rear spoiler, rear tunnel and flaps above the radiator exits. I presume that all of these changes are balanced to increase downforce and handling on Stillen's race car. Do only part of the changes and all bets are off.
Also, I think Stillen's car was set up for the rally in New Zealand, which has a maximum speed limit of well under 100mph. So, high speed changes in handling might not have been their target when making those specific changes to their setup. Just speculation.