Some years ago, I asked at the booking office (Aandalsnes) enroute up to Stavanger about this. I had a coupon, and had paid the reservation fee for a seat - they said ask on the train to pay for NSB Komfort. On the train, my seat was double booked due to a monstrous ___ up (many others too). I asked very politely to pay extra to upgrade to the half empty section - sternly told "Nei". Annoying as I've seen tons of NSB staff over here on coupons too, always looked after them, so didn't hesitate to tell them how **** (one of) their own staff was :P
That's not bad, I'm assuming in the few years since I've used NSB for a long journey the rules on upgrades may have improved somewhat. I'm assuming the £128 is for Oslo-Bergen, that's good value for 3 upgrades plus a full 1st class single.