Just like I had missed the fact that you stated it was Ergo equipped? The Ergo levers are an obvious upgrade as they did not come out until 1992. Chorus went 8 speed in 1991, so I still think that it's a 1990 frameset with the custom option, outside of the Ergo levers. After all, who would want to stick with Syncro?

Personally, I'd probably just stick with 7 speed and go with a modern freewheel with profiled cogs for the improved shifting. That assumes that you can find a ratio you like, as freewheel selection is getting pretty small. I believe 7 speed SIS spacing is compatible with 8 speed Ergo levers, which is what you probably have.