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There may not be a lot of experience with this setup at higher boosts, but I will ask the question anyway.
I am experiencing significant tip-in detonation (pinging) at 12psi, above 5000rpm. I suspect the problem is caused by the transport delay of the extra fuel from the fifth injector reaching the cylinders. One solution is to move the extra injector(s) closer to the cylinders but this reduces the charge cooling benifits.
The fifth injector is currently aligned to squirt fuel across the throttle body inlet duct, forcing it to evaporate before it is drawn into the inlet manifold, improving the mixture distribution but increasing the transport delay. Aligning the fifth injector to squirt dirrectly down the open throttlebody would keep the fuel in suspension longer, allowing it to evaporate quicker, but may bias the fuel distribution to the rear cylinders.
Has anybody successfully run a fifth injector system with the injector aligned with the throttlebody?

I thought 5th injectors were an old thing - isn't it better to just use higher capacity injectors?

And if you are getting detonation, have you considered water/methanol injection?

Or even E85?

My first turbo NA ran a 5th injector....never worked properly and is a compromise at best. It doesnt give a good spread of fuel across the 4 cylinders and I have to question its ability to bring the air temperature down.

An intercooler and a set of big injectors with the pump and ecu tuning to support it will give bigger and more reliable returns.

The fifth injector seems to try to cure detonation (aka preignition) by richening the mixture. The distance from the cylinders means there will always be a lag in getting the fuel in there so you'll get detonation right when you put your foot down on the loud pedal.

I think bigger injectors would be a safer bet. Another cure worth trying would be to retard your ignition timing a few degrees.

The problem with preignition isn't just that the fuel has ignited before the spark, it's that the shape of the shockwave from the ignition isn't piston friendly (shockwave from outside in rather than top down).

Might be worth asking Tom @ FFS:
search.php?keywords=injectors&terms=all&author=Tom+@+FFS&sc=1&sf=all&sk=t&sd=d&sr=posts&st=0&ch=300&t=0&submit=Search

Is this fifth injector from a off the shelf kit, Greddy TD05 perhaps? It has this kind of setup...

93_clubman,
Thankyou for the link. I sent an Email but he maynot be familiar with the problem depending on where his 5th injector is located on his supercharger system.

MxCase,
The injector is located in an AVO inlet pipe, but it is a higher flow injector than AVO use.

My turbo NA mentioned above had the AVO Kit with 5th injector....Honestly not a route worth following.

Are you sure it simply a fueling issue?

I'm making assumptions here, but how are you looking after timing?

With an extra injector I'm assuming you just run a base timing retard, in which case you've probably got a whole bunch of timing still in it at double digit boost. That being the case, the problem is more likely timing than fuel, and while extra fuel can curb the timing problem to some degree, you're likely right on the edge of that if you're not somhow retarding the timing dynamically as boost rises.

I am retarding the spark using the same piggy back processor the controls the 5th injector. It does not have a feature for transient spark control.
There does not appear to be a lot of experience around 5th injectors at highish boosts, so I can either poinner (potentially expensive), change to a different control system and bigger port injectgors (expensive) or settle for a less distructive boost (disatisfying).