There are some super rich tax evaders who can hide many millions from the IRS, but they are few in number and it will cost millions to recover that money after going through all the obstacles such rich people can afford to put in place.
On the other hand, imagine 100 million people all under-reporting $1200 - that's $120 Billion Dollars, or about 6 NASAs. If you actually had to go after everyone, that would be extremely expensive, but if you go after just a small percentage of people, the rest will be scared into properly reporting their income. We all know one or two people who have run into serious trouble with the IRS: enough that we are familiar with the consequences but not enough that it seems like a common and socially acceptable strategy. This is by design.
With the very wealthy, this strategy doesn't really work. Just because you manage to get one bank in switzerland to hand over some data doesn't mean my bank in the caimans is going to do the same. Even if the IRS were to catch up to me, I can afford to wait to cross that bridge when I come to it. You can take billions from certain people and they will still have net worths greater than some nation states - those who can afford to lose can afford to play the game.
It's also worth noting that the tax code has plenty of loopholes that wealthy people and corporations can use to legitimately reduce their tax burden. IRS recovery isn't going to be able to do anything about that. If you have billions of dollars, you can afford an accountant who will make sure there's nothing for the IRS to go after.
Really changing the funding of the IRS is irrelevant - were we to simplify the tax code to eliminate loopholes and make it easier for people who can't afford armies of accountants to properly pay their taxes we could both collect more money and spend less on recovery. It may be impossible to get congress to actually do that, but anyone talking about playing around with the IRS's funding is just trying to look like they're addressing a problem they really have no intention of solving.