Studies of the feasibility and alignment in Hungary of the planned Budapest – Bratislava - Brno - Warsaw high-speed line have been completed
From a link in the article you quote. I can't imagine above 250km/h being viable.The scope of the work is to study route alternatives between the Hungarian and Polish capitals via Bratislava and Brno.
The Hungarian section needs to be studied in detail with proposed track geometry. The minimum line speed is 250km/h but options for 300km/h and 350km/h operation will also be examined.