I'd taken my information from the 1960 LMR (Central Lines) SA, which gives 20 through the junction to and from the station and then 15 between Nos. 2 and 4 boxes through the station itself. However, the 1960 LMR (Western Lines : Crewe and North Thereof) SA gives for No. 2 box 15 through the junction from and to Ashton Charlestown, thus apparently disagreeing with the Central Lines book. By the 1977 issue, now for the northern half of the region, the restriction has become 15 between 7m30ch (ex Victoria) and the 8 milepost.
Regarding possible speeds, the February 2017 presentation to the Permanent Way Institution indicated a possible 80 through Stalybridge and on to Mossley after 75 and then 70 for the curves through and after Ashton. The present speeds of 25 to and from Victoria and 50 to and from Guide Bridge belong to the layout commissioned in November 2012, so just over five years old.
It's interesting that Stalybridge is such a mess because of the railway politics of the 1840s. The end-on junction of the SA&M branch from Guide Bridge with the H&M line from Huddersfield was designed when the SA&M was hoping to amalgamate with the H&M. The M&L built a separate station for its branch. The politics shifted, the H&M was taken over by the LNW which right from the start planned to use the M&L branch into Manchester. A really good junction was planned and powers were sought, but then money spoke and those plans were exchanged for an agreement with the SA&M (by now the MS&L) under which the simple double crossover between the two lines was built, and that was responsible for the lousy speeds there ever since.