185s aren't really suitable for replacing 150s- they're too heavy to keep to time on many routes, and too expensive to run. For example they couldn't feasibly run Huddersfield to Sheffield, a route that is currently operated by 3-car 150s for most of the week and has had platforms extended to accommodate 60m formations. 185s would be too long for the dedicated bay platform at Huddersfield, and would be restricted to 10mph over Paddock viaduct between Huddersfield and Lockwood. Sure, 185s to Northern wouldn't necessarily mean that they'd directly replace 150s on that route, but even an internal cascade would result in a reduction in capacity. Anything with 23m+ vehicles would be restricted to two coaches, as per the 195s which operate the service on Sundays.
The issue of lines with platforms designed for 20m DMUs will need to be resolved (through extensions) as 150s (and the odd 769) are the only 20m vehicle DMUs left, and there won't be any more built, they'll all either be 24m or Stadler odd-lengths. So this will need doing anyway.
I could see a sense in 185s to Northern for use on the originally proposed Northern Connect services, e.g. the WCML routes, or just putting those routes back with TPE as they were a while back. 195s with their high acceleration are proving very useful for Manchester area DMU locals, and it'd be great to be able to release enough of them that ALL their urban/suburban DMU local services used them, allowing some timetable improvements or improved resilience. For instance absent any wires it would be good to bin the 769s off and use 3-car 195s on the Southports and Kirkbys (particularly Kirkby as the acceleration would be a great benefit on the Athertons - they do occasionally show up on the Atherton that terminates at Wigan though). They were ordered for expresses, but in reality their gearbox arrangement renders them superb local units (and they can deal with standing loads well due to the standbacks) - in essence they're more upspecced 172s than long distance units, and would work better on Manchester's equivalents of the Snow Hill lines. Another useful place 195s could be cascaded to by this is the CLC stoppers.
With regard to them being overpowered there is the engine rotation feature, or they could look at removing the engines from the middle coach entirely (there is already electrical crossfeed else the engine rotation wouldn't work) and keeping as spares, though you'd not be able to reduce the weight substantially as the suspension etc will be set up for it, so you may well end up having to put a concrete block in its place.
