Yup here is the thread but no news so far.I'd be surprised if you got 670hp with only stage 4. From what I've seen you'd need 4.25 and possibly a better intercooler to get that horsepower.
Litchfield used to sell sports cats but not sure if that's still the case. Didn't have any about 9 months ago when I enquired. Will be interesting to see what they are now developing re.cats.