Actually, the performance was optimalized a little in the 1.5 patch, though I don't know, because I haven't updated from 1.3_01 yet - still waiting for mods to update. I myself do this to richen the game: I have the Book Mod installed, what gives a new item, the Writable Book. I decided to write a kind of diary, where I'm someone from another world, who was exiled for his crimes to this land by the Arcane Academy, even if he was actually innocent. In this other world, he suddenly noticed that an unseen magical aura causes him a very strong urge to build things he doesn't need, and also fills his head with knowledge how to build them. He is now building a large Cathedral, but he swears, when he's done, he'll start exploring again. Right now, there are a number of mods installed, and some stuff I haven't even built yet, so much stuff I added, like Planes or IndustrialCraft. As soon as this project is done, I'll start exploring, and adding new, random generating mods, like Battle Towers, Ruins, Great Wall, and finally Millinaire, so as the Builder (as I call the my character) explores the world, he finds more and more stuff about the world and why he's actually there.
Not to mention, all the new stuff when the Aether mod is released along with the updated nether mod what comes with it.