David,
The standard RF injection system is 45mm. This will flow up to about 470 - 480 HP before the mixtures get a lot rich (not enough air for the fuel delivered). This car runs well everywhere else and is almost civil for road use.
50mm. / 52mm. (read 2") would take you up another almost 25% in power so let us say just over 600 HP.
The problem comes in at the low throttle positions ie. at tick over and just pottering about. The change in throttle angle will give much more air and therefore need much more fuel and will therefore increase revs etc. A snail throttle set up would improve this as it would change the throttle pedle motion ratio with the butterfly ratio.
I supose the main thing that MUST be considered is 'what am I going to do with the car' and this must be answered HONESTLY. We can all wish that we are going to run the tracks all the time but time is so difficult to get these days.
I have a very good friend of mine that has a GREAT car we built for him. He has a collection of real quick cars and a Suburu in particular that he has tuned and tuned to very large numbers and it is now somewhere between "a hangrinade and unreliable' and I am sure he would agree with this over simplistic summery of the car. As I have said many times before, there is nothing more frustrating than stalling a car because of an engine problem, in this case due to over the top HP for road use.
We all "need" more HP, even my son wanted more power in his go-kart when he raced, but contrary to some people there is a point where too much IS too much.
Best wishes,
Robert