If the flow off the roof doesn't reattach at the trunklid you have negative pressure above the rear creating rear lift
Adding around 35mm of rear spoiler gets reattachment and some lift reduction
Going over 100 mm you get more down-force,but greater overall drag,defeating the purpose of the spoiler
*The spoiler 'sees' some of the more energetic air and captures it's momentum
*An early Firebird spoiler goes all the way to the 'Template'
The Audi TT goes near it,but not quite to it.
*Without a wind tunnel it's hard to know what actually happened with these cars.
*I'd recommend staying with the spoiler if for no other reason,to reduce rear lift,and chance of hydroplaning back there.