I know there are a few people who have a pressure gauge attached to their Peppina and assist the spring by pushing upward on the lever during the pull. I'm wondering roughly how much force one would need to use to bring it up to around 8 bar?

Thanks.

Not much. Stock it provides 4-6 bar. A full manual lever often uses less pressure than tamping to generate 9 bar. In my experience a gentle application of pressure will suffice.