Performance bottlenecks are the bane of developers everywhere, finding ways around them is the hallmark of a capable engineer. I've had a lot of success in my career by pursuing creative and unconventional solutions and wish you the best here.
I'm interested in hearing some more details on your terrain solution. What are you using if not a height map? Modularity is always good, but how is that being applied here? At the end of the day isn't some kind of height data still needed? Good, detailed and efficient terrains are complicated things, I'm super curious how you are approaching this
Edit: page

http://games.joshkirklin.com/sulium
A single player RPG featuring a branching, player driven storyline of meaningful choices and multiple endings alongside challenging active combat and intelligent AI.