I've made it by the side of linked belts mod https://mods.factorio.com/mod/LinkedBelts
if mods["DoubleSpeedBelts"] then
speeds["express-linked-belt"]=4*0.03125
end
but can be done like as
data.raw["linked-belt"]["express-linked-belt"].speed=4*0.03125
Thanks for report, I will do my best.