First, keep in mind that unlocking will notify Asus that you have voided your warranty. Some users have still had hardware issues covered, but others have not.
Second, most of the custom ROMs that I have run tend to be smoother than the stock ICS due to custom kernels and the ability to overclock. Not all ROMs have the best stability, but some do.
For the TF101, Asus is still is on Android 4.0.3 and there are custom ROMs up to Android 4.2.1. I will say that the Jelly Bean ROMs (4.1 and 4.2) are MUCH faster feeling than the stock ICS 4.0.3.
Since the TF300 already has JB, it is hard to say how much faster it will be, but many of the custom ROMs have the stock bloatware removed.