[aerohead]

Here's a couple thoughts.
*Hucho warns us that any roofline contour 'faster' than something like the 'Template' would be prone to generating longitudinal vortices of remarkable drag.
Since the Celica backlight is 'fast' compared to the 'Template' we might presume that you're in an at risk situation.
*Going back to a very thoughtful comment made by Bicycle Bob years ago;if the existing spoiler helps correct the backlight contour it may,if nothing else,break up and kill these strong vortices,rendering a wake of more homogeneous small eddies,which would create less overall drag.
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The 'gaping hole' underneath the spoiler is no doubt a concession to rear visibility,especially when backing,and also,since your in a 4-season environment,allow for snow 'runoff' rather than creating a dam,as with the 1980s Mustang GT 'birdbath' rear spoiler.
I'd be tempted to keep the thing.