I suspect that Paseo spoiler is more likely to help than hurt for a few reasons:
1) It's going to promote cleaner flow separation than the trailing end of the trunk lid. That lid edge isn't as sharp as it could be, and minus the spoiler, there may be downward flow over the trunk lip toward lower pressure, which will increase drag & lift;
2) It's deep enough in the front-to-rear direction that it could permit better flow reattachment coming off the roof if it has separated. If you've got attached flow at the trailing edge, you minimize rear wake.
In a sense, it may be similar to this extreme rear deck treatment from
Autospeed.com:
Australian VL Walkinshaw Commodore
Now the goal of this piece of work was solely to increase downforce (which in this car's case also increased drag). But what makes it similar to the Paseo's is that
if the trailing end of the vehicle is below separated flow, it's useless. By raising the height of that end surface to get reattached flow, you can take advantage of it - to either achieve cleaner separation, which we want, or to generate downforce.
Disclaimer: I'm no expert. I'm just spouting my interpretation of the Paseo based on things I've read.