Ok, this is my original intended reply - fingers crossed it works!
I guess Stena have taken into account the cost of the HSS conversions vs the expected remaining life of the vessels and the actual usefulness (with their limited freight capacity and need for dedicated infrastructure reducing flexibility) and decided its not worth the investment.
Interesting about the Nordica though I note she has 4 engines - 2 x Wärtsilä 18V38 and 2 x Wärtsilä 12V38 which doesn't mean all are in full use all of the time. However according to this site http://www.ship-technology.com/projects/euro_causeway/
the European Causeway which is near sister has 4 Wärtsilä main engines producing the best part of 32kW and a further 2 Diahatsu auxiliary engines. By comparison the Stena Britannica has 4 MAN engines producing just a few kW more. Viking XPRS has 40,000kW of Wärtsilä main engine power onboard (still well short of the Superfasts) whereas Ullysees has a "mere" 31,200kW from her main engines http://www.ship-technology.com/projects/ulysses/
. However, I am getting the Stena Lagan's power output at double the original post/Stena's figures at 21,600kW http://www.naos-design.com/en/design-en/46-p241-lagan-viking-ii-en.html
which seems more likely and leads me to think the Stena site should read 2 x 10800 kW.
Given the age of the design would the Europe's engines not be far less efficient given the advances in technology frrom the late 70's when she was designed meaning more fuel burned per kW of power produced? Also, the Europe is designed to sail at 19kts vs the Nordicas 25kts and the Superfasts 27kts- each additional knot of speed requires a greater increase of power than the last one meaning a Nordica sailing at 18 or 19 knots is going to use nowhere near the amount of power she would at full speed.
It perhaps shows the advances in design/better economies of scale of the Visentini class to compare the original Lagan vikings 15,600 kW to the current Stena Lagan's and particularly the Visentini forwarder (2 x 9450 kW at 500 rpm according to NAOS = 18900kW) to the later larger class such as Stena Lagan (2100 lane metres vs 2885 is over a 25% increase in capacity with less than 13% increase in power
Interesting topic you have started here!