How badly would it effect performance if I was to use code from a pi or other source to implement in a custom framework? Or would it just fuck the server up..?

To be fair, i think if you're asking a question like this, i don't think you have the experise to implement this. Also your question is very unspecific, which 'code' are you referring to?

do it

"PI code" isn't a thing. There is just Java. If the code is bad then the code is bad, no matter what application it is implemented in. You have to take a look at what you want to port over and determine if the performance of that said logic is acceptable.