Depends what you mean.

We have a decent idea of the velocity of money of households at different income levels on the basis of how likely people are to spend all their money vs. holding on to them in ways that may or may not be as effective at stimulating economic activity.

In that sense it is not particularly hypothetical.

In terms of whether people will be more likely to e.g. start a business, that part is a lot more hypothetical. There have been some trials where there seems to have been some effect, but others where it's not clear.

That effect seems very much hypothetical. But that was not part of the classical argument for UBI, and I don't think it's a good idea to use it as an argument for UBI.