Actually, if you really want a huge production boost in the late game, the way to go is adding/editing a late game resource with insane # of spt.
Also, you can easily add + x% production to any late game city improvement; the RaR Crystal Palace IIRC triples base shields already.
Remember that any structure only adds to the base production, that's why a single +10spt tile will change more than several city improvements.
But:
This severely cripples the AI. Period.
And that's why we already may have gone a bit too far in RaR with production boosting. Definitely the reason why RaR is actually easier on the lower level than unmodded.
Why? The human player's best use of those boosters is to turn mediocre cities into production monsters. The AI does not do this; they only build boosters in high-spt cities, where it absolutely is no help for them (since those cities churn out a unit/turn anyway). Ok, they may be able to build a WoW in a turn as well - but, the human will have a prebuild at hand anyway.
So, feel free to change whatever you want, but: You have been warned

.
You will end with an even less competitive AI.