For many year, the Porsche turbo motors have redlined lower than their n/a brothers. It wasn't always the case though. When the Turbo was new in 1976, it redlined at 6,800rpm, while the 911 redlined at 6,250.

It's just a different concept - when turbocharging, there is no need to rev to 7k to make 200hp/L, while it is necessary to rev past 8k if you want over 100hp/L with natural aspiration...