WHP is US figures and is somewhat , well very, over egged compared to ours. 450 horse power at the wheels would put the figure around 560bhp at the fly wheel over here, which isn't possible on the turbos they claim do the figures etc.
My advice, don't use nasioc for tuning info, they have a very strange way of doing things at times and massively over egg results.
So, that said,
what are your actual goals? a well setup 300bhp car will walk all over a 500bhp badly setup car and be alot more fun even with the lower power.
what shell are you using do to it?
how extreme are you actualy going to go? full bare shell with cage, or still in road trim?
What block are you going to use? is it worth getting a closed deck block?
Road and race use are very very different animals, as well are multi lap race vs time attack.