I am sure the IRS will not impede the progress of the funds, but they will make damn sure they collect the tax as soon as it hits US Juristiction. It may be dofferent if there is a specific state law on online casino gambling, in which case rent a property in another state, move there & then cash-in from the casino
In an extreme case, buy a modest house in the UK "up north", get a long stay visa, & have the casino change your address to these & then withdraw. This would allow you to transform the money into a less obvious form for importation back to the US. For that sum, it would be worth it!
From my visit to Vegas, I recall that up to $10,000 can be imported per entry without having to declare the cash at customs. It might be lower for citizens of the US, but eventually you'll get it in. It's a bit like the way we have to work to get cheap cigs and spirits into the UK from Europe, where it is
much cheaper.