I think it's worth noting that there are a multitude of combinations and sizes which all get included under the title of a high flow. I know you guys are talking about one workshop's particular stage 3 high flow but it's worth pointing out that the combination supplied by them will almost certainly be different to any other workshop. A "high flow" turbo can be as simple as putting a larger exhaust housing on an otherwise untouched turbo. This will net bigger peak hp but will sacrifice midrange. If you decide to have a turbo high flowed, ask exactly what is being done. I would be asking for wheel trims and diameters and run the info past somebody who can make some sense of it. There is no point in taking an Hitachi exhaust housing and bolting a 600hp compressor to it. Sure it might make your desired power levels but wont' be much fun.
The last point is that for any desired power output, a high flowed turbo will almost always be less responsive than an aftermarket turbo with matching wheels that's appropriate for that desired output.