I've reduced the maintenance cost of these so that building them (especially the early ones) actually results in a profit not a net loss. I think they still may need buffing however to make them viable over an income building.
If you look in GalCiv3GlobalDefs.XML, there's a GalacticTourismSpendingMod, which is defined to be 0.25 at present. You might try playing around with that before further modifying the tourism structures; I suspect, though I'm not certain, that this controls the base value of tourism. I don't know what base value it modifies to get the base tourism value, but if it's like the trading planet "income" bonus to trade route value then it's going to be based off of production levels, presumably the galaxy total or galaxy average.
Another possible change would be to see how much of a difference it'd make if the structures offered an empire-wide bonus to tourism rather than just a local tourism bonus. That's probably a bit too much of a buff, though.
I've been wondering if its even possible to make a viable tourism planet - eve with extreme tech investment in tourism?
I doubt it. All the tourism improvements I can think of are one-off improvements (there's the three in the Port of Call line and then there's the Restaurant of Eternity, but aside from those I don't believe there are any other tourism structures defined in the base game); as such, they're not really capable of competing with a market world for income generation. At best, they could serve as a way to help offset maintenance costs on factory, lab, and influence worlds or perhaps supplement the income of a market world, but the bonus they provide to tourism income seems far too low for that to be practical.