Anne, syruped brioche doesn’t need to be as soggy as bread that has soaked up soup or dressing. I’ve made the savarin probably 15 or 20 times- 10 of those the first summer I discovered how much I love it- and I would say that it is plenty moist, but stops just shy of soggy. When I make it I cut back on both rum and sugar in the syrup, but leave the water as is. That suits my taste perfectly.
For you, I would recommend starting out with the syrup exactly as written, but reduce it by something like 25%. You could make the full amount but only put 75% on the cake, then save the rest in case you feel it isn’t enough, so you can spoon a little on each slice as you serve it.
For what it’s worth, I don’t think the coffee chiffon in RHC is your cup of tea, so to speak- it needs the syrup to give full coffee flavor but is pretty moist. I liked it but I think as a base for a fairly wet cake I probably like the genoise better. Genoise is drier to begin with than chiffon, maybe that’s why.
What are you thinking of making? Just writing this makes me long for a savarin…