Upgrade to BBK happens for two main reasons: 1) to get extra heat capacity before fading when one does long sessions on track, 2) for looks (yes, because of this reason too
- "OH NOES, still small brakes with not red painted non brand 8-pot calipers in my new shiny 20" wheels?!!"). Twins stock brakes are rather good, they held on track for upto 10min just fine, so for mostly daily driving with slight track usage one should be fine with just brake pads and fluid change. With steel lines & airducts - even better.
There are other reasons two, but imho of less importance in purchase decision. Eg. most aftermarket BBKs of NOT insane rotor size are lighter then stock, for some BBKs consumables, such as pads & rotors, are cheaper then stock.