Power has very little to do with it. The 70s and 80s produced insane turbo engines with crazy, mostly undriveable deathtrap power. That's why most of them were banned, quite quickly, in LMP, F1, WRC, etc.
It's primarily a function of aerodynamics and particularly tyres. As with lots of physics problems, there are practical limits that once reached, require significant engineering effort to achieve small, incremental increases. That's why the fastest production car records were stuck in the mid- to high- 200s for so long, its damned expensive to fund the engineering gains required. In this case, getting over 300mph is mostly about tyres, and that's not a VW specialty, they need a tyre partner to go in with them, Michelin.
Reading some more about the project, there are some incredibly innovative aerodynamic changes to, stuff like where exhaust gases go, small but measurable improvements that add up!
Don't expect the next car to break the record to break it by much.
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