I doubt you will notice any picture improvements using another nowtv device including their nowtv box.
If you find the picture is OK on static images and the picture loses detail and looks blurry and un natural when there is camera moving panning shots such has football etc this could be that you notice the 25fps frames per second rate effect that Nowtv use on their streams.
If you are expecting the same or similar picture quality has BT Sports HD or Eurosports HD via a BT Youview box on the current NowTV streaming service then you will be disappointed.
NowTV have confirmed in press releases that they will be offering 1080p PQ sometime this year, but the press articles was a little sketchy with information about things like 50fps will be included or 5.1 surround sound (where both are important to me).
Or if there will be any additional cost for this higher quality stream as an optional extra to customers if they wished to subscribe to their higher tier PQ service.
Suppose we will have to wait and see what new improvements NowTV bring to their service in the coming months.