Top speed about 160 km/h as said.
I would say 100 km/h is comfortable all day cruising speed although the bike is well capable of more (but you're not, trust me!). The OEM seat will make you stop and take a short walk after two hours. Never had a problem with that.
Wind protection is barely existent (except for the speedometer).
The seat is short and narrow, utterly useless to transport your better half unless she really loves you and it's only for a short flip to the beach.
For travelling two up long distance get something else.
If you wanbt to go fast get something else.
If you want an easy mainenance and reliable go anywhere (slowly) bike get a DR! Get two if you have to take a partner along
I love them. IMHO the most capable and best value for money 650 DS on the market.