I am currently modifying tasks.svgz from the Oxygen theme to fit my needs, and I realized that the blue glow in the task bar is in fact an assembly of 8 base64 encoded PNG images.

I am wondering: is it for better performances, or will a theme with vectorial path have an impact on the drawing speed ? Can it even be done ?

Currently there is no way to modify the color of this element #hover-top easily by editing the SVG file, because it's really just a PNG image.. But if this was a properly defined SVG gradient, it would be much easier to modify.

I'm not sure why it would be done like that - possibly due to bugs/weaknesses in the Qt SVG renderer - which Plasma relies upon to render the SVG files.
In terms of can it be done - the only way to find out is to try i'm afraid.