if you plan to run 20psi + on the 20G you will want pistons...
google EJ257 piston failure, glass piston ej257.
read on rexnet you will find an abundance of info.
I ran 18psi for a long time on my motor with track days with a very safe tune, however others have had similar levels and blown them very easily.
for the extra $600 on a set of forgies you have a shitload more security and room to move, that way you can max the 20G out run it at its efficiency level (22-23psi) and pump 250kw atw out of it. on a stock 257 thats not a very good idea... even MY08 STi onwards are blowing motors at this level.
its your money mate, but i did a lot of reading before modifying mine and i capped my tune off because any higher and it would of popped.
I will put it to Miles and see what he thinks.. Thanks for the info mate, it is muchly appreciated.
I was quoted another $3500 for forged internals though? Where do you get the $600 from?