1.3RC1@13.2kbps is on par with 1.2.1@16kbps on speech.
That's 20% of bitrate reduction. Similar bitrate reduction is observed on higher bitrates (16-40+ kbps)
I was briefly playing around with a speech file I got online (my original copy is a 87kbps VBR MP3(tool in Foobar2000 says 'Lavc56.28')) with v1.3RC vs v1.2.1 I noticed I could lower from 14kbps to 13kbps with v1.3RC and things seemed pretty much the same but going to 12kbps and there was a noticeable hit. although bit rates did not exactly scale down the same from 14kbps to 13kbps to 12kbps...
selected bit rate = actual bit rate...
12kbps = 12kbps (noticeable hit to sound quality vs the two below)
13kbps = 14kbps (seemed to be similar to the setting below)
14kbps = 15kbps (seemed to be similar to the setting above)
basically v1.2.1 @ 14kbps (15kbps real bit rate) is better than v1.3RC @ 12kbps (12kbps real bit rate) on the speech file I tested as the main thing is the v1.3RC @ 12kbps is more muffled where as the v1.2.1 @ 14kbps is not (like the v1.2.1 file @ 14kbps sounds closer to the 87kbps MP3 source file than the v1.3RC @ 12kbps does). is this to be expected?
p.s. this was all done on my Klipsch Pro-Media speakers on the PC with Foobar2000 v.1.3.19.