Question about good design: overriding functions, function parameters or similar

I would be wary about putting too much functionality into functions. Functions cannot be pipelined, so if you need the extra registers to break up a function, you will have to separate the code into two functions - not very reusable.

VHDL 2008 introduced the ability to put generics on packages and declare generic types and functions on anything. But I dont think this will be supported by the synth vendors any time soon (its supported by sim tools).