As Blunty says really ^^ I've always done the breathing first with my cars. But it just so happens when doing it to a Vauxhall V6, you need to spend more to get more out of them. Exhaust, filter and remap usually isn't enough for these engines so like i've done Exhaust, Filter, Gas flowed Heads, Uprated manifolds and downpipes/headers, Enlarged Throttle Body, Smoothed Inlet piping then a Remap.