The Aix-en-Provence to Avignon 320 km/h section on the LGV Méditerranée, is a "test section", I believe it was introduced so that SNCF could monitor the effect on the track and trains of running at 320 km/h against 300 km/h in regular service I think the 320 km/h section was inaugurated before the opening of the first section of the LGV Est as well. The maximum speeds of TGV sets are as follows:
TGV Sud-Est: generally 300 km/h, though some are 270 km/h max, when introduced into service in 1982 their maximum speed was 260 km/h
TGV Atlantique: 300 km/h
TGV Réseau, Duplex, Réseau-Duplex, Dayse, POS, Thalys etc all 320 km/h. The Réseau sets were 300 km/h max until they were refurbished and put to use on the LGV Est. The TGV Duplex were also 300 km/h max until they started running at 320 km/h on the Aix-en-Provence to Avignon 320 km/h test section.
As the LGV Sud-Est is primarily used by the double-deck TGV sets, though the TGV Réseau also operate on through services into Italy, I would say that all the TGVs operating on the LGV Sud-Est have a 320 km/h top speed, though the speed limit on the LGV Sud-Est is 300 km/h max.