Yes, I'm sure that any attempt to make general-case script-run world generation would run into performance issues pretty quickly. You can't make any assumptions about shortcuts, and so are forced into a brute-force or worst-case approach too often.
See, I think about it completely backwards from your point of view. I would gladly give up TC and BB generation. I'm not trying to achieve block-for-block fidelity. I just want to capture the essence of the terrain character. In my view what would be needed is for RTG to make their terrains externally available so that a biome config could specify generator=rolling_hills or generator=snowy_mountain or whatever names you use internally. Then the surface/filler/mix blocks and decoration objects create the variety over that basic terrain shape. Essentially, instead of team-RTG choosing the realisitc biome, the config would tell RTG which generic realistic terrain algorithm to use for a given biome. How do-able that is depends a lot also on how much has to be hard-coded for a given terrain algorithm to work with smoothing, transitions, beaches, etc. It's certainly a lot of work either way, and I don't know if that sounds like where you guys want to take RTG. I think it fits within the scope, but may not be the right direction. Certainly performance would suffer, and might make it all unworkable regardless.
Really I started investigating BiomeBundle looking for an alternative to BOP biomes, which I have begun to find tiresome over the course of their development. It seems like the unique ones were dropped or their generation parameters were changed to the point that they feel interchangeable rather than distinct. Dunno. Maybe it's just color choices. I can't really put my finger on it. They do seem a little preoccupied with dead stuff.
Anyway, I'd much prefer to use RTG terrains than TC if I could somehow get TC's biomes into RTG. But I'd need realistic versions of the biomes somehow, and I don't even know if TC puts them into the vanilla biome array at all. I'm not clear on how it handles biomes <256, but not vanilla. Obviously it has to convert biomes >256 to something <256 for saving to the files, but it's not clear if something like 125 is accessible to a mod like Geographicraft if TC isn't generating the terrain. The number is valid, of course, but a TC terrain doesn't really have any generation parameters outside of TC worlds, so I'm not sure how vanilla would generate 125 if it existed in the normal biome array. Maybe it just adds it to the biome array, but not the generation groups, so it would never be generated in the world except by TC.
So, after all that, just want to say that i LOVE Geographicraft and Underground BIomes. They are an indispensable addition to any sort of worldgen. Maybe somebody will find a way to make my dream happen. It could even be me. :-)