Yeah, there's a ton of research that's been done on this.
Money can't make you happy but not having enough money creates an incredible amount of stress. Once you have enough money (I think generally around $110,000 a year but obviously changes on location) to meet your needs then having more money doesn't make you happier. Mo money mo problems and all that.
It's just another excellent argument for taxing billionaires at higher rates, we're doing it for their happiness.
I don't think taxing billionaires as individuals really helps that much.. What I think would help is making the taxes on non-value adding activity, like passive investment into derivative markets at least on par with the taxes on personal income. There shouldn't be more incentive in doing nothing, than doing something.
Honestly, I'd just be happy with Billionaires paying the same tax rates as the rest of us... so closing up taxation loopholes like the passive derivative market investments and bank loans on stock, etc. People shouldn't have extra options just because they're more wealthy - equal opportunity tax avoidance, haha.
Companies should pay the same too. Make it all flat.
That would make it an absolutely massive improvement from the current situation.