Upping the contribution limit without upping the benefit would just be a hidden tax on the upper middle class, but not the rich, who do not work for salary. If you want to tax the rich, you need to raise the upper tax rates.
It's exactly the opposite of hidden. It literally shows up as a separate item on your taxes. However, I don't think most people actually realize that it's a regressive tax that goes away when you make more money. Though society is at a place where a lot of people don't care if elderly people starve and freeze to death just as long as the upper class get slightly lower tax burdens.
You need to implement social security taxes on investment income, not raising the SS taxable cap on wages, to accomplish this. It makes sense to not overly tax burden wage income (productive effort) while taxing investment income (rent seeking) at a higher rate, no?
I'd just go with adding all income (investment or otherwise) into a total pool and then tax that with a single set of rules instead of this stupidly complex tax system.