You don't care about lag, so we can bolt on a turbo that has a HUGE compressor section, for unmatched efficiency at high boost levels. You have a choice to make: Stock location, or rotated mount turbo? The stock location turbo keeps costs down, and you can use your existing up pipe and downpipe. The rotated-mount turbo involves a custom up pipe and downpipe, and in most cases as external wastegate, but clocking the turbo inlet to the side makes for improved flow and allows for a larger inlet pipe.
On the stock location front, we've got the Blouch Dominator turbos. The Dominator 2.5 XT-R (affectionately called the "Polka Pickle") boasts a horsepower capacity of 490 at the crank, but still bolts in to the stock location (barely) and has a built-in internal wastegate just like your original turbo. Note that with most larger stock-location turbos, you'll need to have removed the TGV system, because the compressor housing is going to occupy the space where the TGV sensor used to live.
For an example of a rotated-mount turbo setup, take the AMS 750R as a good example. It's sold as a kit that includes the up pipe and downpipe, and your choice of external wastegates. Figure a minimum horsepower capacity of 500 at the crank, and potentially much more depending on fuel, boost level, and what size and configuration turbo you choose.