you can think of it this way, the b13/b14/b15/p10/p11 trans was designed to support a 145 crank hp motor from the factor.
The spec v trans/gearbox which if im not mistaken is the same design and gears other than ratio differences and case except bell housing differences as the VQ fwd vehicles which supports a 287hp v6. So the design of the gears was made to support that much power.
So of course its internals are going to be beefier. Its common knowledge and is visible just looking at the differences.
I think the 6 speed will hold up quite well with the 500+whp sr's