You can vacuum bag and probably be OK as long as you take into account the differences in the end product. Carbon is very unforgiving and fails rather catastrophically. There is a big difference often between real world laminates and engineered calculations or lab samples that needs to be addressed. More refined manufacturing techniques can help eliminate these. The builder/buyer just needs to be cautious.
On the question of sail, you are spot on. If the sail does not match the mast carbon or not, you will be slow. The same goes for aluminum. This is why I am opposed to those nice tuning guides where they tell you the tension and rake and setting for each condition. You need to set your mast setting to match your sail. Each sail will vary slightly. Different makers can vary significantly and we are F boats so this is allowed. Different masts or mast from different batches also have shown huge differences in their properties. Different crew weight combos or sailing style will pay in as well. What works for 1 boat will not necessarily be right for the boat right next to them.