Save your doh and do it yourself. Just my 2 cents. The motor has SOOO much potentially. Back when the IS first came out, there was a magazine with 3 of them on the cover and what their HP was. Something like 303, 450, and 615, but it was a while ago. Shit, I'd take any of 'em.
you can't do the is430 yourself--unless you want to do a complete engine & tranny swap.
the rmm is430 is NOT the stock engine or drivetrain, its a v-8 engine
but you are correct that the stock is300 engine has the potential to put up good #'s (but so does every car really)