After doing a lot of research I stumbled over this thread, listing benchmarks for a lot of microsdhc cards.
xda microsdhc testthread
The card currently in my Kobo is actually pretty good already, with it's 0,3 mb/s random write speed.
Many cards offer substantially worse results in that area, because they are developed for fast sequential r/w speeds, not random access, so chances are high you get a worse card performance wise, if you just pick a new one randomly based on a higher class.
The "Sandisk Mobile Ultra Class 10 UHS-1 microSDHC" line is apparently one of the better variants, with theoretical 4-5 times the access speed.
I ordered myself a 16gb variant (for 11€) and let you guys know if I notice any difference in reality after it arrived.