I think there is no evidence that increasing the capital gains taxes a bit would have a negative effect on the economy, as there is not much evidence that decreasing the capital gains tax has had a positive effect on the economy.
What we do know, for example, is that decreasing the capital gains taxes led to more inequality. There's almost causal evidence for that. However, if we look over history, our periods of best growth were when we had higher capital gains. If you compare across countries, it's not because you have higher capital gains taxes that you have lower growth. It's very hard to establish that evidence. I don't think there's strong evidence for this.