the RPI will beat the Akrac hands down sound and power wise and its cheaper to import from RPI(you might get stung by custom though ).
also evolve one seems ever good as well (and cheaper), but dont know if you can get them anymore as they were made by Car Planet Racing ( they have gone bust owing creditors thousands ££ ).
if it was for me RPI and evolve and then akrac (because it no value compare to the other two ).....but if you can get one 2nd hand for 2000ish then i go for the akra...but if sound is your main concern RPI all day long (I had their gt system on my old E60 M5 ..its was awesome and i know my exhausts as i have/had alot of high powered japanese rockets
)