Never been on a Voyager, but when they were loco-hauled by class 45/46/47 in the 1970s/80s/90s charging at 80mph at the bottom with 10 on, they would be doing around 34mph at the top. I couldn't put my hand on 35/42/43 hauled logs but I have some double headed Warships somewhere from 1968/69. I have only been down the Lickey behind Westerns. Notable was a run from New Street to Cheltenham stop in a few seconds over 36 minutes behind 1066 Western Nobleman in 1975 with a late running train off the 25kv to Plymouth. We touched 100 mph over the River Avon at Defford.
HSTs were only a few mph faster than loco hauled despite the higher power to weight ratio.
Once, on a charter from Gloucester, a class 40 struggled to make 20mph at the top.