This long disproved concept that the Bolton route is too slow keeps coming back time and time again for some reason. A Manchester to Preston service timed for a Class 185 calling at a single stop (Bolton or Wigan) is historically two minutes faster via Bolton. This was before the Bolton corridor was essentially rebuilt so the time savings running via Bolton will probably grow even more thanks to 100mph running etc. To summarise: the route via Golburne Junction is slower and longer.
I didn't say that the Bolton corridor was too slow. It's not. And as you say, with the improvements already done and (hopefully one day) electrification from Wigan - Lostock, running a sub-30 minute service via Bolton should be more than possible.
The present semi-fast Wigan-Bolton-Victoria service, which extends to Stalybridge, would be just as fast as the Chat Moss trains but for the pootling along at the end of the route. Stopping at Salford Crescent & Central, with a 3/4 minute wait at one of them, presumably because of congestion at Victoria, makes the service slower. And unfortunately, there is no westbound fast service via Bolton at present, with everything making additional stops between Bolton and Wigan.
More importantly, though, running hourly fast services with the first stop in Wigan actually helps ease overcrowding by separating the Bolton and Wigan commuters onto different trains. And, as I see it, running via Golborne is the easiest way to do that without stealing a path from a Bolton service.