has anyone had a 350 mag mpi stock motor on the dyno to see where the max hp. was made i read that 4800 to 5200 is the max rpm but is that where the power is or is it the same say at 4600 to 4800

That seems a little high to me . I would think it would be around 3200 to 3600 is where the torque is . And yes max hp will be at 5200 . Just my .02

i guess my question is i have twin 350 mpi and my max rpm with 23 marage plus props is 4700 can i gain anymore top end speed if i changed pitch and got 5000 rpm

I would try one size smaller prop and see what you get . The only way to know for sure is to try it .