If Neville Hill gain all 12 4 car 331s and retain the 16 333s this will provide an increase of 4 EMUs, which will allow some slack in the fleet as it's quite common for cancellations to occur in the peak due to unit shortages. Providing additional services must also be difficult with the single line sections near Apperley Junction and through Baildon as well the busy junctions around Shipley.
However it may mean some West side services can be run as 6 car 331s which would be welcome on the Bolton corridor
I suspect a bit of extra capacity could be gained by stopping more Leeds-Skipton services at Apperley Bridge & both those and Leeds-Ilkley at Kirkstall Forge, meaning that they would be less likely to trip over the all stopper Leeds-Bradford FS. It wouldn't be popular slowing the Skipton services down (when Kirkstall Forge opened it was originally served by the Ilkley services so not as controversial), but sometimes they do trip over services anyway and its not unusual for them along with the other Aire & Wharfe services to end up queuing outside Leeds. So maybe it might help punctuality.
However outside of the peaks there probably isn't quite enough demand for additional services on these just yet, and some additional capacity will be made available from Bradford & Shipley once / if the LNER 2-hourly services commence. But in the peaks an extra couple of services might be possible before the current additional services. The single line section up to Guiseley does present a challenge but it is just about possible to thread extra services through it. I don't envisage any need for extra Bradford-Ilkley services, so the Baildon section would remain the same. Of course there are still the Carlisle / Morecombe services to consider, though someone did suggest combining these at Leeds and splitting up route, so maybe this could be a possibility, and of course the freight services.
So it would be a tight squeeze, but there are additional Leeds-Skipton & Leeds-Skipton runs in the high peaks so it can be done & a couple more runs of each would help offer the capacity lost with not running the 2* 3 car 331s.
This was more than an assumption. Northern's stated plan was for there to be 16 (I think) 3-car 331s to be based at Neville Hill. These would have enabled replacement of 321/322s (including Doncaster services) and allow for some 6-car services to run between Leeds - Skipton.
Yes, I do remember that now. However I think these days to assume that any plans initially made will happen exactly as stated is being overly optimistic. So until plans actually start to come into play, its probably safer to keep them as assumptions.