I tried converting a FLAC album to MP3 V4 (I deliberately chose a poorer bitrate, typical of what might be the rare lowest-quality audio I'd find in my library) and then ran the dynamic range plugin on the converted album. Here's the results:
FLAC:
DR Peak RMS Duration Track
--------------------------------------------------------------------------------
DR15 -0.16 dB -20.86 dB 15:16 06-Les Préludes
DR16 -1.62 dB -24.56 dB 15:36 01-Symphonie fantastique: I. Rêveries - Passions
DR14 -5.44 dB -24.86 dB 6:23 02-Symphonie fantastique: II. Un bal
DR19 -0.04 dB -28.23 dB 16:15 03-Symphonie fantastique: III. Scène aux champs
DR13 0.00 dB -18.15 dB 4:55 04-Symphonie fantastique: IV. Marche au supplice
DR14 0.00 dB -20.32 dB 10:35 05-Symphonie fantastique: V. Songe d'une nuit du sabbat
--------------------------------------------------------------------------------
MP3 V4:
DR Peak RMS Duration Track
--------------------------------------------------------------------------------
DR15 -0.36 dB -20.86 dB 15:16 06-Les Préludes
DR16 -1.31 dB -24.56 dB 15:36 01-Symphonie fantastique: I. Rêveries - Passions
DR14 -5.37 dB -24.86 dB 6:23 02-Symphonie fantastique: II. Un bal
DR19 -0.05 dB -28.23 dB 16:15 03-Symphonie fantastique: III. Scène aux champs
DR14 0.00 dB -18.15 dB 4:55 04-Symphonie fantastique: IV. Marche au supplice
DR14 0.00 dB -20.32 dB 10:35 05-Symphonie fantastique: V. Songe d'une nuit du sabbat
--------------------------------------------------------------------------------
There are only slight differences in the calculated peaks.