158's are often 3 cars as TP were and CT as well as GW have split them up to form 3.
The East Mildands ones used on the Liverpool-Norwich services aren't though, they're all two car units. Though I agree that the change from 170s to 158s on this route hasn't really been a backward step, as following the recent EMT refurb the 158s look very smart and fit for purpose, and now that EMT have enough units to allow 2x158s on Liverpool-Norwich on many services, capacity has increased over that provided by a 3 car 170.
The problem for routes such as Liverpool-Norwich and Manchester to Scotland is that they don't have a single unified authority to fight for service improvements for them; as is mentioned, Liverpool-Norwich traverses a number of PTE areas.
Birmingham-Chase line services fall largely within the Centro boundary, and so the PTE is more inclined to show an interest in the operation of the service, not least because Centro provide subsidy for the Rugeley services.
Birmingham always seems to have been incredibly lucky with it's rolling stock for local services in the last 20 years, and it's a real godsend that Centro chose 150s instead of Pacers! A very wise decision! It does seem peculiar that the West Midlands has the option of deploying high quality Turbostars on local routes with relatively low patronage when there are many more suitable longer distance routes that desperately need upgraded rolling stock.
It's also surprising how little opportunity GMPTE has to invest in increased and better quality rolling stock around Manchester, given that Manchester's overcrowding woes are considerable in comparison to Birmingham. Does seem a tad unfair!