Honestly, I think the main impact would be a genuine interest by the Romans to conduct military expeditions in that direction. No great invasions, but establishing control further and further south, securing trade.
After all, with a map that takes you that far south you can have people look for gold, iron, wild animals, slaves, all sorts. I'd go so far as to suggest that even with the difficulties of the Cataracts and the Sudd, just being able to travel that way opens new areas for the Romans to exploit, and so eventually set up small waystations and forts to guard those routes.
This could have two VERY different impacts concurrently.
1) Disease. So much disease exchange. Whilst it isn't massive quantities of trade, it isn't safe for the Romans, and in turn, the Romans introduce new diseases into Africa. If they eventually balance out and Sub-Sudd Africa develops an immunity to Roman diseases and vice versa, then this massively expands both of their worlds.
2) The "Rhine Forts Effect" - effectively settlement. The Romans are inevitably going to need to buy supplies of food from the locals - shipping it all from Egypt would just be mad. So just like in the Rhine Valley, loads of nearby people can potentially make themselves quite wealthy not just trading goods, but food to these forts. Whether this leads to improving local agriculture, adapting Roman techniques, or hybridising them, I'm unsure - but I can certainly see it leading to a taming of the Sudd in some way.
If that route can be maintained, you could potentially have new ideas lead to increased urbanisation in the Lake Victoria region earlier.
