Because the RB20 needs all the response it can get, you'll lose that with larger cams.
Also porting is a waste of money on a 20 head. They flow around 30% less than a RB25 head anyway.
Please read the 260rwkw RB20 thread (linked via the FI Guide). You'll see people have tried various setups and the setup that is the best for 260rwkw overall?
TD06 Greddy KIT, stock RB20, decent tune
People dont bust their 20 gearboxes, they don't destroy motors either
People tried cams, plenums, different manifolds, cam gear settings etc.
Overall these items - some of them big $$$ items – made no difference and in some cases the setup actually LOST power and/or response.
Being a pioneer is all well and good, but if you are on a budget, why bother? People post up all this information to help everyone else for free, I say use it