There is no problem with the MX5 / Miata 5 speed box under 250 HP.
I know of no evidence that proves the 6 speed box which is a bought in component by Mazda on the MX5 has any more capacity for power transmission then the 5 speed made by Mazda box.
NC miata 5 speeds grenade at around 250HP, but I have never heard of a 6 speed grenade under 300HP. I have heard they can handle far more.
