I am running a Magnex downpipe, and a resonated centre section with a WRX STi backbox on my MY94 WRX.

The car is mapped to 16psi and runs very well, and the Sti backbox is fairly quiet, which is good for my purposes.

How much power would I gain if I switched to a Ninja or scoobysport/ Magnex back box? Any ideas?

 

Hardly anything.
If you run a decatted system the vast bulk of your extra power comes from your decatted downpipe..a decatted centre section will add aliitle something too.
Having said that, i run a full decat straight through system with an angled exit Afterburner Vortex. When i replaced my Ninja with the Vortex there's no doubt i noticed a difference in power.....just as Scoobyworld who sell the system claim. See Scoobyworlds web site or speak to them.
Be warned...the Afterburner Vortex is very loud on a full decat....so probably not what your after.
My Classic goes on a rolling road at the end of the month along with my mates, so it will be interesting to see the results as the decatted tek3 cars are identical apart from the fact he has a Ninja b/b and i have the angled exit Afterburner.

 

Revolution Motorstore (www.revolution-motorstore.co.uk) make there own stainless steel exhaust with a choice of round or oval tailpipe. I've recently purchased the full straight through system and it has given a noticeable increase in power, especially in lower gears.
They've tested this exhaust and the results show between 32 and 40BHP increase!

 

I've already got a straight through. Just want to know the effects of the backbox alone.

 

I have a Ninja and centre de-cat. No rolling road yet but mucho grunt since fitting. Sounds the dogs danglies!!!!!

 

IIRC John Banks did a test when he was running about 400bhp on a 2 litre. The difference between a Blitz nurspec and an OEM backbox was ~8bhp even at that power level. For people running lower powers or swapping from one straight through backbox to another, the difference will be smaller.

JB may be around to correct me at some point. If he does, at least he knows someone sometimes listens to him!

 

It can make a difference: http://dyno.scoobynet.co.uk/uk/andrew_tang2.htm

The only difference between the last two runs are the centre section and backbox.

Everything else about the run was the same, ambient temperature, map on the ECU, tyre pressures, fuel (to the best of my knowledge), same rolling road, etc.