My bet is that it is 450 - 500 bhp possible but the RS500 intercooler might be the weak link, I believe that you can get 30 degree lower intake temps with the spec R equivalent which is actually smaller and then allows more air to the front rad or air filter. (based on What Mark Shead has said and written many times)

My engine was held back by the intercooler at 478bhp (mine is an RS500 one) and the next step really is to port the head, swap the cam and then pocket the pistons to suit. I am on a Borg Warner twin scroll.

I would imagine that whoever is going to live map it will know much more than I can tell you though.