No one can say exactly how many lines of C code, or any other high level language, you can fit. 2000 might be an average (I don't know) but exactly will depend on the code. Some lines are compiled into one instruction, some into many. I've run out of code space plenty of times on PIC's.
However, if people are saying that around 2000 is the limit, then you might well have a problem with 2500. If you are writing code directly you might be able to optimise for minimal code, but probably not using flowcode.
There is no way to add extra program memory to your PIC, sorry.