There is a vast world of difference between a good and bad job!
A bad job can see you with less power, just because a company gets good results on say a VW head doesn't mean they will get good results on an alfa head, you need to make sure they have worked on and. Can prove results for your type of engine.
Ideally you want
T/b to inlet port matched
Inlet to head port matched
Head to exhaust port matched but so that the opening tapers slightly away from the head (smaller) to speed up the flow of gases.
Them you can get in to all sorts of extras like haveing the valve collers trimmed back etc.
You also need to look at things like gasket material between the t/b and inlet and head, these days it's common for them to be upgraded to ptfe as this has better thermal barrier protection and so cuts down heat transfer between parts, and in turn this reduces inlet temps and cooler temps etc better more powerful combustion.
Then you can look at the shape and design of the butterfly valves in the t/b , it's common practise and has being for many years to trim the spindle in half, fit counter sunk screw and machine a chamfer on to the leading edge of the plate.
Of course all this costs money although some can successfully be done at home, but couple it with a good cons air feed to the air filter a better less restrictive air filter in the standard housing and an exhaust should see you on the road to a few extra horses.