[Kawman]

Gus,

The difference between the SRX and the SR is, the SRX uses the aluminum heads as cast (480hp). For $600 more, the heads are ported and polished, getting another 30hp from improved airflow and that is the SR (510hp).

The R engine uses a bigger cam and you go from a dual plane intake to a single plane intake. Now you get 550hp, but at a bit of a cost in terms of a more pleasant street driver.

What do you need the extra 40hp for when you already have 510hp in a 2,400lb car? Don't get me wrong, I'll happily sell you whichever you want, but my advise would be to go for the SR.

I've got one in stock if you need it.