My cranktrain was dynamically balanced for £50
on equipment that was intended for turbine blades (5x more accurate than the usaual gear).
Dont go to BBT for pistons! if you want some forged ones, i'll give you the details of where to get mine (Accralite...very good). as for conrods, H section rods are available, but doubt they are necessary for 7500rpm.
You have to remeber that it isnt just the bottom end thats limiting the max rpm, its the ability of the head (cams/valves) to flow at high rpm, the combustion chamber design to promote a fast enough burn to allow it, and of course the valve train for keeping it all together, and not destroying itself when you wind it up (lighter parts...less inertia forces blah blah blah).
You can also look at line boring the main journals so as to keep it all true, adn of course boring the cylinders to ensure a true bore, and good finnish.
If you want really high rpm, it would be necessary to have a steel crank that is counter-balanced to your reciprocating mass (piston+rod assembly)....assuming that the head flowed well enough to allow that sort of engine speed of course.