Im sure Dynpack measures in BHP not WHP its 513bhp not 513WHP. So thats more like 435whp. 10% transmission loss is no where near, its more like 15%.
How can they calculate transmission loss if they only had your block and no gearbox or actual car attached to it? Or was it done when you got it back in and fitted?
Agreed with the C16 it would make X more power due to timing advance but you're way above what that figure, if its BHP then i'd say its not a million miles away but still too high.