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Late to the party, but I also would have said "lose that aftermarket spoiler" to reduce drag.
There's a chance the OEM spoiler may have reduced both rear lift and drag compared to the "base" car that had none. It'd be useful to know if Cd figures are available for the two versions (turbo and non-turbo versions, if I'm not mistaken).
As for noticing a difference in driving once the conversion is done, I'm afraid you won't be able to conclusively / reliably say whether it helps or hurts your energy consumption in regular use - likely way too many variables to eke out the effect of a single mod (unless it's utterly massive).
You'll have to do something a little more methodical, such as repeated coast-down testing from high speed, or terminal velocity / average speed testing on a steep, long hill.
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