But that makes no sense.
I presume that the Tourism value is originally set to something like 0.01 - that is, you get 1% of your total influence area in tourism income. It can't be much higher than that, given what I'm seeing for values in the game.
If the value in the improvement is a Flat value of 0.01 (which is what it originally way), then that's a 100% bonus (0.01 + 0.01), since you've doubled what the original Tourism value was. Which is why a MULTIPLIER of say 5% would be better. (0.01 * 1.05) = 0.0105. The way it currently works, the current values are severely OP.
(and my 0 for Paxton's isn't actually a multiplier of 0. Look at the XML - it's just that Paxton's doesn't have a Tourism adjustment, so you stick with the normal value).
And are you sure it doesn't need to be enabled by getting the Tourism tech? I don't remember that it shows up UNTIL that tech is researched (or you get the Paxton's Emporium wonder completed earlier). There absolutely has to be some trigger for tourism to start, because it 100% isn't there for at least the first several dozen turns.
The only current Galactic Wonder that gives a global bonus is the very last one, the Restaurant of Eternity.
A current game example: I haven't researched Tourism yet, but I've built both Paxton's Emporium and the Import/Export Center on my homeworld. I'm getting well over 15 credits/turn just in tourism on that homeworld, and the influence ring isn't very big (it's about turn 40 or so, so we're talking about an influence of maybe 10 hexes in radius, at most).
Meanwhile, the Gross Income modifiers are ludicrously underpowered. I've got a ton of adjacency on those two, and the sum total of the Gross Income modifer for both is... +8%. or, less than half of one basic Market Center. Really, why bother?
Look at the XML, and diff it against the Retribution file to see the changes.
In fact, I just looked at the MasterTechDefs.xml file. Two things stick out:
The tech called MasterInterstellarTourism has this stat:
"CanTourism", with an EffectType of "faction", and a Flat bonus of 1. That looks explicitly like toggling a boolean value from 0 (false) to 1 (true). Which leads me directly to believing that tourism is NOT in any way enabled until that tech is researched.
Also, look a the tech called MasterKrynnConversionTech
It has a the following: a Tourism modified, GLOBAL in scope, targeting all Colonies, with a MULTIPLIER of 0.25. That is, a +25% bonus to Tourism for the Krynn.
This is exactly the kind of thing I'm suggesting we do with the Improvements.