Under the consumption tax HR 25, any individual pays his FULL tax burden when he buys stuff. So when the drug lord buys stuff, he pays his full share... not just part of it.But the people buying from him won't be paying any tax on that money, income or sales tax. Think of it this way. The income tax gets the money going into the underground market, the sales tax get the money going out.
It's a good thing you said "... may be worsse at collecting ..." since that also means that it may be better, too - which is what I believe to be the case. There has been a good bit of conspicuous consumption of expensive items in he US by those in the underground economy which can be seen from government confiscation sales and I think you'd have an uphill battle convincing most people that IT was paid on the funds used for those purchases.
Additionally, it isn't at all clear that the IT gets the money "going in" as you say since a good chunk of drug purchases are supported by stolen funds, not earned income.