There are upgrades out there. Conestoga is the main player for upgrades. There was a lot of aftermarket support for them 10 years ago, but now they are pretty much old tech and a lot of the distributors disappeared. You can still get a few different turbo kits, larger injection pumps, better injectors, etc. Google the oilburners forum. There's guys over there that eat, sleep, and breathe these old engines.
My bus has a 6.9 IDI. It's slow to get going. Not sure the power level(it's slightly over stock) but it still runs good. It will eventually get up to 70, however it will be at red line when it finally gets there. Your cruising speed will depend on what your bus is geared at.
There's only a few things you need to watch out for. Coolant condition is one of the major ones because these engines seem to suffer badly from cavitation. So make sure the additive and ph levels are correct. You also might run into bad glow plugs/module too, they're not too hard to replace. Only use Beru/motorcraft plugs. For the module you can use a toggle switch and bypass it altogether. Other than that, just perform routine maintenance and it should last pretty long.