Errr... wut?
Any reason to not *demand* some sort of soft API (slot signature) instead of digging for the next best thing that remotely looks like the right thing?

Maybe provide hard API like "Effect::performFeature(Feature f, QVariant v)"?

Actually that was an experiment. I didn't want to use QMetaObject::invokeMethod with a string, so I wanted to try whether there's another way.

But maybe adding another method is the better solution, it's not really beautiful code I wrote there.

maybe only "perform" (and have the signature fill in the information), but in general a much better solution ;-)

EffectWindow* ?

whitespace

EffectWindow* (no space)

Please check the coding style. There is supposed to be a space before * and &. See both Qt style https://wiki.qt.io/Qt_Coding_Style:
"For pointers or references, always use a single space between the type and '*' or '&', but no space between the '*' or '&' and the variable name: "

Example:

// Wrong
char* blockOfMemory = (char* ) malloc(data.size());

// Correct
char *blockOfMemory = reinterpret_cast<char *>(malloc(data.size()));

And KDE coding style at https://community.kde.org/Policies/Kdelibs_Coding_Style reads:
"For pointers or references, use a single space before '*' or '&', but not after"

Unfortunately not with such a nice example as in Qt case.