The Motul 300 is indeed a bunch more viscous than
any of the MTF spec fluid's viscosities. (15.2 cSt @ 100*C for the gear 300 vs. ~6.8 to ~7.3 cSt @ 100*C for the MTFs)
At this point, I would only use the 300 if I were open tracking the car in the south/southwest/deserts in the summer.