That's not what I was informed by the project mech engineers. Firstly Mk3b carriages would have been much cheaper to build than Mk4s, both on a development cost and a per-unit basis. The cost of design swing plug doors had already been paid for by the CIE/442 work. Also, apart from the design of the bogies, the Class 89 was described to me as being very similar to a class 90. As the prototype development work had paid for the bogie design, 31 Class 89 should have been cheaper than paying to design a brand new loco, bogies and drive system for Class 91's. The reason why we got 91s/Mk4 is that the specification was upped from trains with a maximum of 125mph to 140mph to improve the COBA score. There was no way 89s/Mk3bs were going to meet this spec. I am guessing that the person who did the COBA score may have miscalculated the cost of signalling adjustments which is why we never got past 125mph in service. There was some actual work done on the track side, looking at the section from Grantham to Peterborough to fit in the new 90mph 158s with the 140mph 91s - it wasn't as simple of assuming that it was the same as with 75mph 156s and 125mph HSTs.
The irony is that with an extra 1500hp on the maximum rating Class 89s would have been better for the current 125mph service, even taking into account the 240hp reduction in continuous rating.