For saving game data, i usually go into /data/data and find the corresponding folder to the game, copy that with root explorer onto my sdcard. Load new rom, install game, go back to /data/data, delete the new folder, and then move the old folder from the sd card to /data/data. Has worked with every game i have tried it on.
EDIT: I tried this on angry birds, moving from BB 0.5 to MIUI 4.1.1 and it did not work. What i had to do was copy the file "highscores.lua" from /data/data/com.rovio.angrybirds/files to my sd card, then install the game, press play, and then exit the game, now paste the highscores.lua into the files folder.
EDIT2: Okay, that shows the saved scores, but when i go to play, it wont let me play the next level that should be unlocked, and if i play the previous one, it shuts the app down when i beat it. Seeing if i can figure it out now.