I've installed PR1.2 (full flash, clean device). Wow. Ping-times are now 5ms-10ms without further configuration. Browsing by wifi now feels very much more snappier than ever before. No more weird delays.

This might all be due to my FB7270 router, but given the clear fact it now works much better I'd say it was de N9's software as well. Haven't tried SIP, but this sure is promising.