Alright... Paul referred in his PM to the XY Problem, so let me start from the beginning. 
Problem X: For my project, I want to use a large (~6cm) 4 digit display, preferable 7-Segment. I'm using already 12 of the digital pins of the Arduino for inputs.
So as I understand, I need to use I2C to control the display to avoid controlling every digit/LED separately - am I missing anything?
Now I found the HT16K33 LED controller driver, which seemed to be suitable for my needs. According to the data sheet of the Kingbright SC23-12EWA it has common cathode:
I would connect all cathodes of the 4 digits with COM0 of the driver, all segments with the ROW-connectors.
Accoding to the HT16K33 data sheet, the maximum voltage output seems to be VDD - max 5.5V.
As I need a supply of actually 8.2V for the Segments + V(current limiting resistor), I somehow need to boost the voltage.