I worked in north northampton for a bit. What you say about Kingsthorpe and surrounding area is true. Main roads gridlocked and residential streeets as rat-runs.Maybe a better investment would be a tram from the centre partially using the old rail line then branching east up boughton rd to the uni.
The Kingsthorpe problem is a consequence of local authorities meddling with the traffic and allowing housebuilding (particular thanks go to Daventry & South Northants on this, who keep approving schemes on Northampton's boundary without having to worry about the impact) without actually dealing with the root cause.
If you look at the A508 corridor or the A5199 corridor (the old railway ran between the two) and say you want to head to the M1, then you're forced to go into Northampton - which is why places like Kingsthorpe, Dallington and Kings Heath are gridlocked now, because they're no longer handling just local traffic. And before anyone says 'well reinstating the railway will fix that' - guess again, people may be heading for the M1 to then head on the A43 (towards Oxford) to the A45 (towards Daventry) or for Bedford or Luton which a link from Leicester to Northampton won't touch.