The IRS is pretty awesome and incredibly underfunded, yes.
But we could simplify the tax code... The bigger the surface area, the easier it is to find a vulnerability / loophole. And anymore, the tax code is basically being written by BigCos to protect their interests and provide breaks for themselves.
Enforcing a tax code that's 20-feet tall when printed out when companies are as complicated as they are -- it's a wonder anyone pays taxes. It would take years in court to figure out what a company honestly owes.
If it's simple -- you make x profit or y revenue -- then you pay z dollars -- for any public company, good luck grossly cheating that.
People always ask this as if it's impossible to change laws or have agencies that actually want to follow through on their missions. As if corporations are so devious and brilliant they are unfindable. It's just not true. If you vote for people who actually want taxes, the enforcement takes care of itself. It works the exact same the other way, because these people gut agencies and replace them.