As far as I tested the results were pretty similar(Highpass filter->overlay / Sharpen filter).
Do they basically do the same thing?
Actually nevermind. I forgot I asked this exact question back in the day and got an answer. 
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Can you link the older thread about this, so that other people will also know?
Actually it was not posted on this site(it was on the KDE forum), and the answer from the user named Snudl was this :
"Yeah, in general sharpening filters increase the contrast of the small (local) details in an image. The type of algorithm used for finding those local detail is often a convolution / kernel filter: Kernel (image processing) - Wikipedia
In short, it compares every pixel in the image with its neighboring pixels and then calculates a new pixel value from it, and from that it can enhance (sharpen), extract (edge-detect) or reduce (blur) the local differences in the image. To find larger details it has to consider a wider radius of surrounding pixels (larger kernel size), which also takes longer to calculate.
A high-pass filter essentially gives you the separated details in a layer, so using a blending mode like Overlay, Linear Light or Grain Merge will act as sharpening, while for example Grain Extract (the opposite to Grain Merge) will result in blurring. The reverse is also how you create your own high-pass filter: Add a Filter Layer above your image, choose Gaussian Blur and play with the radius, then set its layer blending mode to Grain Extract."
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