Piracy is not theft. Piracy is piracy. The question is whether piracy is a sin or not. Not theft.
The first thing to note is that supposing piracy were a sin, it would be less serious than theft. Theft takes away something from someone who owns it to give it to someone who doesn't have a right to it. Piracy merely gives to someone who hasn't purchased a "right" to it. There's no taking away involved.
Second, note that even "theft" is justified in cases of necessity. The classic example is that a starving man can morally "steal" food. This is because he has a moral right to the food. (In which case it is not technically theft any more, but that's a whole different autistic tangent.)
Lesser degrees of necessity than literal imminent starvation can also justify "theft." Namely, any instance in which a good's "owner" would be unreasonable if he refused to give that good to a given person if asked for it, is an instance in which that given person could justifiably "steal" it.
Piracy involves no property, in the strict sense. There is no "taking away". Only an addition. So piracy and theft can by no means be equated.
It is true that the collective effect of many instances of piracy might have negative effects on the production of "intellectual goods" as a whole. This does not however affect the moral meaning of piracy, since morality concerns individual choices, not trends as a whole and obscure economic consequences.
This is also telling against the argument that "the workman deserves his pay". No one denies it, but who exactly should pay? I can work as hard as I please writing great novels or making beautiful music, but no one will pay me for that. What they will (sometimes) pay for is the ability to access these goods. Which is not, you will note, the work itself. Is this system unjust? No money paid for the work, but plenty of money paid for the marketing and convenience? Some would say it's just a stupid system.
However, the potential for negative macroeconomic effects of piracy is what justifies governments in creating laws concerning the fiction of "intellectual property". These laws are prudent when they actually, in real life, serve their intended end, and imprudent when they do not. In no case that I can imagine would such laws be immoral.
The degree of fidelity a Christian owes to the laws of a government is not absolute. The government's laws are subordinate to the true common good. If in a given instance, following the law would harm the common good, breaking that law in a way that serves the common good is justified, because it would be serving the same end which the law exists to serve. This is assuming that the given law in this instance is not itself contrary to the natural law.
Making the prudential decision as to whether breaking a law in a given case is justified is generally out of the "moral jurisdiction," so to speak, of individual citizens. No one should take it upon himself to decide if breaking the law in a given case is a good thing, because the presumption leans so heavily against it. The only exception to this rule is cases where only that person himself and people under his authority would be involved. One might for instance settle a family problem internally without recourse to the police, even if we suppose there were a mandatory reporting law in such cases.
That is about the extent of my personal analysis of the issue. Corrections gratefully accepted.