If your issue is the social problems associated with extreme inequality, it's ultimately just a question of whether the tradeoff is worth it, rather than whether it grows the GDP faster or slower. There are a million ways we could restructure society, with a million consequences for the economy and the social good. It's all a matter of picking the tradeoffs we can all live with.

That being said, wealth redistribution doesn't necessarily need to mean less private sector capital investment. If you take a billion dollars away from someone who would have invested it and give a thousand each to a million people (whether through highway robbery or through raising taxes here and lowering them there), there's no reason they can't invest it all. Hell, they could invest it in a fund managed by the former billionaire, and you'd have the same amount of investment managed by the same people.

That situation is unlikely, but it also discounts the value of investments besides the sort that billionaires tend to make. While some redistributed wealth might be reinvested or 'wasted' on consumption, there are also be a lot of small investments that happen when you share the wealth. Children eat better, which improves their futures in pretty much every way. People start more small businesses, bodegas and farm stands and hair salons and startups. People spend money on education, for themselves and their children.

That sort of investment is also vital to our economic future, no less than the capital liquidity provided by the billionaires.